Updated the schema JSON format, schema comparison and tests

[VisLab]

Fixes the conflict with attributes in outputted JSON format.

[Copilot]

Pull request overview

This PR updates the JSON schema output format to omit empty list attributes rather than serializing them as empty arrays. This fixes a conflict in the JSON format where attributes like suggestedTag, relatedTag, valueClass, and unitClass were being written even when empty.

Key changes:

• Modified JSON serialization to conditionally include list attributes only when non-empty
• Updated existing tests to verify empty lists are omitted, not present as empty arrays
• Added comprehensive new tests for empty list omission and extras section roundtrip

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
hed/schema/schema_io/schema2json.py	Changed build_attributes_dict to check if list attributes are non-empty before adding them to the output dictionary
tests/schema/test_json_explicit_attributes.py	Updated test_empty_lists_omitted to verify empty lists are completely omitted rather than present as empty arrays
tests/schema/test_schema_format_roundtrip.py	Added three new comprehensive tests: test_json_empty_list_attributes_omitted, test_extras_sections_roundtrip, test_library_schema_extras_roundtrip, and test_library_schema_score

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