[tool] fix: handle empty image inputs in ToolAgentLoop

[denismegerle]

What does this PR do?

Fixes a compatibility issue in ToolAgentLoop, downstream image-processing pipelines require passing passing images=None (instead of an empty list) when a tool call returns no new images.

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Test

Not covered by an existing upstream test in this PR. Change is a narrow input-normalization to apply_chat_template(images=...) when no images are produced.

API and Usage Example

No API change.

# No API changes. Behavior change: images passed to chat template is None when empty.

Design & Code Changes

• In verl/experimental/agent_loop/tool_agent_loop.py, normalize images to None when new_images_this_turn is empty.

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[gemini-code-assist]

Code Review

This pull request addresses a compatibility issue in ToolAgentLoop by ensuring that None is passed for the images argument to apply_chat_template when no new images are returned from a tool call, instead of an empty list. The change is correct and directly implements the described fix. This prevents potential issues in downstream image processing pipelines that may handle None and empty lists differently. The implementation is clear and confined to the intended scope.