Update user_content initialization to None, handle System only event in Evaluation

[kauabh]

Change user_content initialization from empty string to None for better clarity. Added condition to skip evaluation when user input is None, as this is required for Evaluation. This closes issue #3760

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Link to Issue or Description of Change

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

• Closes: 3760
• Related: #issue_number

Please describe the tests that you ran to verify your changes. This is required
for all PRs that are not small documentation or typo fixes.

Unit Tests:

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

Please include a summary of passed pytest results.

Manual End-to-End (E2E) Tests:

I created a simple Root agent. Executed this adk web. After one interaction I chnaged "user" role from store database to "model". Without the proposed changes it code was failing as expected while creating evaluation set, after code changes it was running fine.

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

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[gemini-code-assist]

Summary of Changes

Hello @kauabh, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the evaluation generation process by improving how absent user input is handled. It changes the default initialization of user_content to None for better semantic representation and introduces a mechanism to bypass evaluation for invocations lacking user input, thereby preventing irrelevant test cases and enhancing the robustness of the evaluation system.

Highlights

• Improved Clarity for User Content Initialization: The user_content variable is now initialized to None instead of an empty string, providing clearer semantics for cases where user input is absent.
• Skipping Irrelevant Evaluations: A new condition has been added to skip the generation of evaluation cases when user_content is None, as such cases are generally not useful for testing or grading.
• Issue Resolution: This change addresses and closes issue Evaluation Invocation.user_content ValidationError when session has invocations without user events #3760, which likely involved scenarios where an empty string for user_content caused unexpected behavior or was semantically incorrect.

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[kauabh]

/gemini

[gemini-code-assist]

Code Review

This pull request correctly refactors the user_content initialization to use None instead of an empty string, which improves type safety and code clarity. Additionally, it introduces a necessary check to skip evaluation for invocations lacking user input, preventing potential errors and focusing evaluations on meaningful interactions. The changes are sound. My review includes a minor style suggestion to remove an unnecessary whitespace-only line, in accordance with PEP 8.

[gemini-code-assist]

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[kauabh]

/gemini review

[gemini-code-assist]

Code Review

The pull request updates the initialization of user_content to None for improved clarity and introduces a condition to skip evaluation when user input is absent. This change addresses the stated goal of handling cases without user input for evaluation purposes. A potential improvement has been identified regarding the handling of empty Content objects.