fix(evaluation): skip invocations without user events to prevent ValidationError

[Oxygen56]

Description

Fixes #3760

EvaluationGenerator.convert_events_to_eval_invocations iterates over events grouped by invocation_id. For invocations that contain no user-authored event (e.g., internal background turns, system-driven tasks where all events have author != "user"), the code previously left user_content as an empty Content(parts=[]).

Invocation.user_content is typed as a required genai_types.Content. While Content(parts=[]) does not cause a Pydantic validation error, creating an Invocation with no meaningful user content is semantically incorrect — an invocation without a user turn is not meaningful for evaluation.

This fix: initializes user_content = None and skips the invocation if no user event is found. A DEBUG log line is emitted so operators can diagnose unexpected skips without producing noisy warnings.

Changes

• src/google/adk/evaluation/evaluation_generator.py: initialize user_content = None instead of Content(parts=[]); skip invocations where no user event was found.

Testing

• All existing test_evaluation_generator.py tests pass (all existing test cases include user events, so they are unaffected).
• The fix corrects the behavior described in Evaluation Invocation.user_content ValidationError when session has invocations without user events #3760: sessions with invocations that have no user-authored event no longer produce invalid eval invocations.

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[adk-bot]

Response from ADK Triaging Agent

Hello @Oxygen56, thank you for contributing to the adk-python project and submitting this pull request!

To help us review your PR more efficiently, please ensure it complies with our contribution guidelines:

1. Contributor License Agreement (CLA): It looks like the CLA status check is currently failing. Please make sure to sign Google's CLA at https://cla.developers.google.com/ to proceed.
2. Logs / Verification Screenshots: Since this is a bug fix, could you please provide a log sample or screenshot showing how the fix works after application? This helps reviewers better understand the impact.
3. Unit Tests & Test Summary: Please write or update unit tests under tests/unittests/ to cover this new edge case (skipping invocations without user-authored events). In addition, please run the tests and provide a brief summary of the passed pytest results.

Thank you so much for your effort and support!

[Oxygen56]

Fixed the commit author to a GitHub-linked email. Triggering CLA recheck.