fix(_function_parameter_parse_util): return [] not None for empty schema properties

[devteamaegis]

What's broken

_get_required_fields() in src/google/adk/tools/_function_parameter_parse_util.py returns None (bare return statement, line 437) when schema.properties is falsy. The caller in _automatic_function_calling_util.py assigns the result directly to declaration.parameters.required. Any code that then iterates over .required raises TypeError: 'NoneType' object is not iterable. This fires for any function whose properties dict is empty — for example, a function that only takes tool_context once it's been added to ignore_params.

Why it happens

return with no value in a list[str]-annotated function returns None instead of the expected empty list.

Fix

Line 437: changed return to return []. One character change.

Test

Added test_empty_properties_required_is_empty_list_not_none in TestBuildFunctionDeclarationLegacy. It calls _get_required_fields with an empty-properties schema and asserts the result is [] and is iterable without raising.

Fixes #5920

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[adk-bot]

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[rohityan]

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