fix(aws_ecs_metrics source): skip over empty ECS metrics payloads

[tustvold]

Summary

The ECS metrics endpoint can produce null or empty objects for stopped containers in version 3 and version 4 respectively. This currently causes the parser to fail

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

How did you test this PR?

Added test

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

Notes

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References

[pront]

Thank you @tustvold! Please add a fix changelog since this is a user facing change.