Fix to_sharegpt optional block rendering "None" for missing extra columns

[vineethsaivs]

What

to_sharegpt(..., merged_prompt=...) lets an optional [[...]] block reference more than one column, e.g. merged_prompt="Location: [[{city}, {country}]]". Only the first column in a block gates whether the block is rendered. When the block is kept, str.format is called with every referenced column, so a later column whose value is None renders as the literal string "None" in the emitted training text:

# merged_prompt = "Location: [[{city}, {country}]] end"
# row: city="Paris", country=None
# ->  "Location: Paris, None end"      # the literal "None" leaks in

Single-column blocks are unaffected, because the sole column is also the gate and is emptied before format is ever reached.

Fix

Coerce None to "" when building the format arguments for a kept block, so an absent extra column renders as empty ("Location: Paris, end"). The first-column gating semantics are left unchanged.

Test

tests/python/test_to_sharegpt_optional_none.py extracts the pure-Python formatter builders (_parse_combined_prompt, _create_formatter) via ast, so it runs on CPU with no import unsloth (same pattern as tests/python/test_change_system_message.py). It fails before the fix ("Location: Paris, None end") and passes after, and asserts the all-present, empty-gate, and single-column cases are unchanged.

[gemini-code-assist]

Code Review

This pull request updates the chat template processing in unsloth/chat_templates.py to coerce missing (None) columns to empty strings ("") within optional blocks, preventing them from rendering as the literal string "None". Additionally, a new test suite tests/python/test_to_sharegpt_optional_none.py has been added to verify this behavior under various scenarios. I have no feedback to provide.

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