Fix AssistantTracker.is_active() returning False after activation with empty lists

[jnMetaCode]

Summary

Fixes a bug in AssistantTracker.is_active() in chat_template_utils.py.

After activation via activate_tracker(), _rendered_blocks and _generation_indices are set to list arguments which may be empty []. The is_active() method checks truthiness (return self._rendered_blocks or self._generation_indices), which returns False for empty lists, incorrectly indicating the tracker is inactive.

This means calling activate_tracker with empty lists, then checking is_active(), returns False even though the tracker was activated. This breaks the re-entrance guard in activate_tracker which relies on is_active() to detect double activation.

# Before:
def is_active(self) -> bool:
    return self._rendered_blocks or self._generation_indices

# After:
def is_active(self) -> bool:
    return self._rendered_blocks is not None or self._generation_indices is not None

Test plan

• Verified _rendered_blocks is initialized as None and set to list on activation
• Verified is_active() is used as re-entrance guard in activate_tracker
• Existing tests should continue to pass

[Rocketknight1]

Yes, LGTM! Thanks for the fix.

[HuggingFaceDocBuilderDev]

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