fix: change condition in remove_thumbnail_asset to resolve key error

[botanical]

Summary: Summary of changes
Addresses VEDA-287: Resolve Key error in veda_dataset_pipeline

Changes

• Updates condition to check if assets is not None before calling payload.pop("assets")

PR Checklist

• Update CHANGELOG
• Unit tests
• Ad-hoc testing - Deploy changes and test manually
• Integration tests

[anayeaye]

We need to update line 24 too, so that we can test if assets exists

assets = payload.get("assets", {}) should be assets = payload.get("assets", None)

EDIT
I think having an assets key at all is a problem for the existing pipeline because the pipeline and stac schemas have conflicting assets meanings--so this level of logic about other non-thumbnail assets seems unhelpful. @ividito @botanical should we just check if assets is in payload.keys() and then if it is payload.pop("assets")?

[smohiudd]

I agree that the current logic is awkward. I also think we should just pop the assets from the payload.