fix: Return empty list instead of None from _get_required_fields for schemas without properties

[ItsMacto]

Please ensure you have read the contribution guide before creating a pull request.

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: BUG: _get_required_fields returns None instead of [] for empty properties, causing TypeError #5920
• Related: fix(_function_parameter_parse_util): return [] not None for empty schema properties #5921 (earlier PR proposing the same change, stalled on an unsigned CLA since May; this is independent work with regression tests)

2. Or, if no issue exists, describe the change:

Problem:
_get_required_fields in tools/_function_parameter_parse_util.py is annotated -> list[str] but has a bare return when the schema has no properties, so it returns None. Any caller that relies on the declared contract (e.g. iterating the result, as reported in #5920) gets TypeError: 'NoneType' object is not iterable. A maintainer confirmed the bare return as the culprit in the issue thread and endorsed return [].

Solution:
Return [] instead, honoring the declared return type. One-line change.

One honest scoping note: the two current callers in _automatic_function_calling_util.py guard the call behind a truthy properties dict, so today the empty branch is only reachable by calling the util directly (a function whose sole parameter is tool_context produces parameters=None rather than required=None). The fix corrects the contract violation at its source and hardens the util for present and future callers; the regression test pins the contract at the util level, and an added declaration-path test documents the tool_context-only behavior.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Three tests added to tests/unittests/tools/test_build_function_declaration.py:

• _get_required_fields returns [] (not None) for a schema without properties — fails on the unfixed code (assert None == []), passes with the fix.
• _get_required_fields returns the correct non-empty list for a schema with properties (guards against over-correction).
• A tool_context-only function's declaration has parameters is None (documents the actual public-path behavior).

# before fix:
FAILED test_get_required_fields_no_properties_returns_empty_list - assert None == []

# with fix:
$ pytest tests/unittests/tools/test_build_function_declaration.py -q
41 passed

pyink --check and isort --check-only are clean on both touched files.

Manual End-to-End (E2E) Tests:

Not applicable beyond the unit level — the change is a pure return-value correction in a private utility with no runtime surface of its own; the declaration-path test covers the integrated behavior.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

None.

[ItsMacto]

/gemini review

[adk-bot]

Response from ADK Triaging Agent

Hello @ItsMacto, thank you for creating this PR!

This PR is extremely well-structured and fully complies with our contribution guidelines:

• It links to the associated issue (BUG: _get_required_fields returns None instead of [] for empty properties, causing TypeError #5920).
• It provides a thorough testing plan along with unit test results.
• The CLA check has already succeeded.

I have labeled this PR with the tools label because it addresses tool function parameter parsing (tools/_function_parameter_parse_util.py). I have also assigned @xuanyang15 as the shepherd for this component.

Thank you for contributing!

[ItsMacto]

Note on the Mypy check failures: the flagged "new" errors are in src/google/adk/models/lite_llm.py, which this PR does not touch. The baseline-vs-PR error diff appears to be unstable for that file (it also only fails on 3.10–3.13, passing on 3.14). All unit tests, the linter, and A2A checks pass. Happy to rebase/retrigger if that helps.

Approved by mistake

[rohityan]

Hi @ItsMacto , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Your PR has been received by the team and is currently under review. We will provide feedback as soon as we have an update to share.

[rohityan]

Hi @wuliang229 , can you please review this. LGTM.