[GCS Destination] JSON output omits null fields, add option to include nulls

[marcoaanogueira]

Describe the problem

When using the GCS destination connector, fields that have null values are being omitted from the output JSON.

This causes downstream systems that rely on a consistent schema (e.g. expecting all fields, even if null) to break or require workarounds.

Steps to Reproduce

1. Create a source with optional fields that may have null values.
2. Configure a connection with the GCS destination (JSONL format).
3. Run a sync where some records have null values in optional fields.
4. Observe the output files in GCS, fields with null values are missing entirely.

Expected Behavior

The GCS destination should have an option to include fields with null values in the output JSON.

Example of expected record:

{
  "id": 123,
  "name": "Test",
  "email": null
}

Instead, the current behavior outputs:

{
  "id": 123,
  "name": "Test"
}

Context

This behavior breaks compatibility with systems expecting a fixed schema.

Suggested Solution

Add a configuration option to the GCS destination connector (e.g., include_null_fields: true) that controls whether null fields are kept in the output JSON.

Additional context

Connector: destination-gcs
GCS destination connector version: v0.4.8
Format: JSON Lines (jsonl)

Let me know if you need help testing or validating this feature, happy to contribute!