[Model] Fix a check for None but the return value was empty list in Gemma3 MM vision_embeddings

[hfan]

Purpose

No functional change.

It is best practice to check properly for empty list if the value can be an empty list.

Also in tpu_commons when we try to trace the graph without MM inputs, this check fails and causes the graph for text-only input still tries to trace the MM code path.

Test Plan

Existing CI should still pass

Test Result

[github-actions]

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[gemini-code-assist]

Code Review

The code change adds a check for an empty list of vision embeddings before calling prepare_attn_masks. This prevents a potential error and improves the robustness of the code. The change is approved.

[gemini-code-assist]

Checking the length of vision_embeddings prevents a potential error when prepare_attn_masks is called with an empty list. This condition ensures that attention masks are only prepared when there are actual vision embeddings to process, improving code robustness.

if vision_embeddings and len(vision_embeddings) != 0: