fix: prevent dropping grounding-only responses in BaseLlmFlow

[hemasekhar-p]

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Problem:
ADK Java BaseLlmFlow drops a model response that has grounding metadata but no content, because the "skip empty response" guard in buildPostprocessingEvents does not check groundingMetadata. adk-python keeps such responses. This is a Python-parity gap that can silently lose grounding metadata (e.g. Google Search / citation grounding) on responses that carry no other content.

Solution:
Added && updatedResponse.groundingMetadata().isEmpty() to the skip condition in buildPostprocessingEvents. This ensures that grounding-only response (no content) should produce one emitted event. Also added unit

Testing Plan

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:

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• All unit tests pass locally.

Added the postprocess_onlyGroundingMetadata_returnsEvent regression test in BaseLlmFlowTest.java. This test simulates a live API chunk by injecting a LlmResponse payload containing only GroundingMetadata. It successfully verifies that the postprocess method emits the event (returning a list of size 1) instead of incorrectly dropping it.

Checklist

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• My pull request contains a single commit.
• I have performed a self-review of my own code.
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• I have added tests that prove my fix is effective or that my feature works.
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