fix: handle None llm_description in PptxConverter (fixes #1534)

[Muhtasim Munif Fahim (Muhtasim-Munif-Fahim)]

Fixes #1534 — PptxConverter crashes with an UnboundLocalError/TypeError when the model returns None for image captions.

In _pptx_converter.py, changed the alt text assembly to coerce None variables to empty strings before joining them:
alt_text = "\n".join([llm_description or "", alt_text or ""]) or shape.name

Added a mocked test case in test_module_misc.py (test_pptx_null_llm_caption) that verifies markitdown survives a None return from llm_caption.

[Muhtasim Munif Fahim (Muhtasim-Munif-Fahim)]

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