fix: handle None return from llm_caption in PptxConverter

[octo-patch]

Fixes #1534

Problem

llm_caption() is typed as Union[None, str] and can return None in two cases:

• When base64 encoding of the image data fails (exception caught internally)
• When the LLM response's message.content is None (some models can return this)

After the try/except block, the code does:

alt_text = "\n".join([llm_description, alt_text]) or shape.name

If llm_description is None, this raises:

TypeError: sequence item 0: expected str instance, NoneType found

Solution

Add or "" to the llm_caption() call so that a None return value falls back to the initialized default of "":

llm_description = llm_caption(...) or ""

This is a one-character change that makes the behavior consistent: whether llm_caption raises an exception (caught by except) or returns None, llm_description remains an empty string.

Testing

The fix is minimal and targets the exact line reported in the issue. The downstream str.join() call now works correctly in all cases:

• LLM returns a caption string → used as-is
• LLM returns None → treated as empty string (same as exception path)
• LLM raises an exception → llm_description stays "" (existing behavior)