fix(pptx): coerce None shape/notes text to '' before concatenation (#1808)

[haosenwang1018]

Issue

Closes #1808

Root cause

PptxConverter.convert performed unguarded + concatenation on shape.text and notes_frame.text. python-pptx returns None for either value when:

• A text frame has a run with no <a:t> child
• Third-party deck generators emit chart/SmartArt elements whose title text is None

A single such shape failed the entire .pptx conversion with TypeError: can only concatenate str (not "NoneType") to str.

Fix

Coerce shape.text / notes_frame.text to "" before concatenation at both call sites in _pptx_converter.py. Normal decks see no behavior change — only the None case is different (was: raise; now: empty contribution).

This is the exact code shape the issue body proposed.

Tests

• test_pptx_with_none_shape_text_does_not_crash — uses the issue body's monkey-patch trick to make TextFrame.text return None for a specific value, then converts a deck containing it. Pre-fix raises TypeError, post-fix completes and the title is preserved.
• test_pptx_with_none_notes_text_does_not_crash — covers the second call site (notes slide).
• test_pptx_normal_text_still_converts — regression guard for the happy path.

$ pytest tests/test_pptx_none_text.py
3 passed in 0.39s

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