adding support for None chat templates.

[NicoGrande]

Description

Avoid type errors when template_config resolves to None. This can happen if no chat_template_path is provided, if the chat template fails to load correctly, or if no chat template is passed in.

Tests

Added unit test for this case.

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/trainers/post_train/rl/train_rl.py	0.00%	6 Missing ⚠️

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