[Fix] Leave empty objects in metadata

[kstribrnAmzn]

Issue #, if available:
#212

Description of changes:
This change updates how Shadow document metadata is calculated. For a simple document updates which remove only top key value pairs, the behavior will not change. For a more complicated document where nested objects are removed, the behavior is updated to keep the empty parent object (which matches IoT Shadow).

Why is this change necessary:
By removing empty objects from the metadata the shadow manager is calculating the ShadowDocMetadata.desired (or reported) field to null which results in a later NullPointerException when building the updated metadata later.

How was this change tested:
Added unit tests as well as performed manual testing against the case mentioned in the issue.

Any additional information or context required to review the change:
It's useful to know how Jackson JsonNode's function

Checklist:

• [N/A] Updated the README if applicable
• Updated or added new unit tests
• [Pending] Updated or added new integration tests
• [N/A] If your code makes a remote network call, it was tested with a proxy

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[kstribrnAmzn]

Some manual testing logs snippets. In this case I created a shadow document and then set a field to null to remove it. For the case of removing a simple value node (aka JSON value) the shadow document is updated to

GetThingShadowResponse(payload=b'{"state":{},"metadata":{},"version":24,"timestamp":1759530985}') : {'payload': '<payload>'}.

Now for the case of removing a child key-value pair, it is shown that the parent structure maintains.

GetThingShadowResponse(payload=b'{"state":{"desired":{"created":{}},"delta":{"created":{}}},"metadata":{"desired":{"created":{}}},"version":22,"time
stamp":1759530294}') : {'payload': '<payload>'}.

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[kstribrnAmzn]

Some notes from manual testing I performed. There were 4 tests cases I tried - they were executed serially to build off each other. Test cases 2-4 are also covered by unit tests now.

1. Create a shadow locally with a child object
2. Remove the child object from the shadow (this is the error in (ShadowManager): NullPointerException on shadow desired state field deletion #212)
3. Add a new, unique child object to the same parent object
4. Delete the child object from the previous step while adding another unique child object

I added a few logs to the shadow manager to help me verify this but the logs below should be fairly explanatory. I logged at warn to ease finding the logs:

// Creating the object
2025-10-10T20:27:00.922Z [WARN] (AwsEventLoop 1) com.aws.greengrass.shadowmanager.model.ShadowStateMetadata: [Kody] Final desired state. {Patch Desired={"mystate":{"mysub":{"timestamp":1760128020}}}, Metadata Patch={"desired":{"mystate":{"mysub":{"timestamp":1760128020}}},"reported":{"uuid":{"timestamp":1760128020}}}, Updated Desired={"mystate":{"mysub":{"timestamp":1760128020}}}}
 2025-10-10T20:27:00.922Z [WARN] (AwsEventLoop 1) com.aws.greengrass.shadowmanager.model.ShadowStateMetadata: [Kody] Final reported state. {Patch Reported={"uuid":{"timestamp":1760128020}}, Updated Reported={"uuid":{"timestamp":1760128020}}}
// Read back through dummy component
2025-10-10T20:27:00.980Z [INFO] (Copier) com.example.ShadowOps: stdout. GET LOCAL (CREATED): GetThingShadowResponse(payload=b'{"state":{"desired":{"mystate":{"mysub":"123"}},"reported":{"uuid":"f130d5a4-1dce-4fae-8f52-b395a2ded88a"},"delta":{"mystate":{"mysub":"123"}}},"metadata":{"desired":{"mystate":{"mysub":{"timestamp":1760128020}}},"reported":{"uuid":{"timestamp":1760128020}}},"version":1,"timestamp":1760128020}') : {'payload':...

    
// Remove the child
2025-10-10T20:27:00.990Z [WARN] (AwsEventLoop 1) com.aws.greengrass.shadowmanager.model.ShadowStateMetadata: [Kody] Final desired state. {Patch Desired={"mystate":{"mysub":null}}, Metadata Patch={"desired":{"mystate":{}}}, Updated Desired={"mystate":{}}}
// Read back through dummy component
2025-10-10T20:27:01Z [INFO] (Copier) com.example.ShadowOps: stdout. GET LOCAL (REMOVED): GetThingShadowResponse(payload=b'{"state":{"desired":{"mystate":{}},"reported":{"uuid":"f130d5a4-1dce-4fae-8f52-b395a2ded88a"}},"metadata":{"desired":{"mystate":{}},"reported":{"uuid":{"timestamp":1760128020}}},"version":2,"timestamp":1760128020}') : {'payload':...

// Add new child
2025-10-10T20:27:01.006Z [WARN] (AwsEventLoop 1) com.aws.greengrass.shadowmanager.model.ShadowStateMetadata: [Kody] Final desired state. {Patch Desired={"mystate":{"addedfield":{"timestamp":1760128021}}}, Metadata Patch={"desired":{"mystate":{"addedfield":{"timestamp":1760128021}}}}, Updated Desired={"mystate":{"addedfield":{"timestamp":1760128021}}}}
// Read back through dummy component
2025-10-10T20:27:01.026Z [INFO] (Copier) com.example.ShadowOps: stdout. GET LOCAL (ADDED): GetThingShadowResponse(payload=b'{"state":{"desired":{"mystate":{"addedfield":"123"}},"reported":{"uuid":"f130d5a4-1dce-4fae-8f52-b395a2ded88a"},"delta":{"mystate":{"addedfield":"123"}}},"metadata":{"desired":{"mystate":{"addedfield":{"timestamp":1760128021}}},"reported":{"uuid":{"timestamp":1760128020}}},"version":3,"timestamp":1760128021}') : {'payload':...


// Remove + Add child
2025-10-10T20:27:01.035Z [WARN] (AwsEventLoop 1) com.aws.greengrass.shadowmanager.model.ShadowStateMetadata: [Kody] Final desired state. {Patch Desired={"mystate":{"addedfield":null,"newerfield":{"timestamp":1760128021}}}, Metadata Patch={"desired":{"mystate":{"newerfield":{"timestamp":1760128021}}}}, Updated Desired={"mystate":{"newerfield":{"timestamp":1760128021}}}}
// Read back through dummy component
2025-10-10T20:27:01.046Z [INFO] (Copier) com.example.ShadowOps: stdout. GET LOCAL (EXCHANGED): GetThingShadowResponse(payload=b'{"state":{"desired":{"mystate":{"newerfield":"winner"}},"reported":{"uuid":"f130d5a4-1dce-4fae-8f52-b395a2ded88a"},"delta":{"mystate":{"newerfield":"winner"}}},"metadata":{"desired":{"mystate":{"newerfield":{"timestamp":1760128021}}},"reported":{"uuid":{"timestamp":1760128020}}},"version":4,"timestamp":1760128021}') : {'payload':...

For the desired values I added the following logs here. The reported logs are similar.

        final JsonNode patchDesired = metadataPatchWithRemovedFields.get(SHADOW_DOCUMENT_STATE_DESIRED);
        if (!isNullOrMissing(patchDesired)) {
            desired = nullIfEmpty(merge(state.getDesired(), desired, patchDesired));
            logger.atWarn()
                    .kv("Metadata Patch", metadataPatchWithoutRemovedFields.toString())
                    .kv("Updated Desired", desired.toString())
                    .kv("Patch Desired", patchDesired.toString())
                    .log("[Kody] Final desired state");
        }