fix: PptxConverter tolerates None shape.text and notes text

[alvinttang]

Refs #1808.

PptxConverter.convert does unguarded + / .lstrip() on shape.text and notes_text_frame.text. python-pptx returns None for a text frame whose <a:r> run has no <a:t> child, and for certain third-party-generated decks. One malformed shape then fails the whole file with AttributeError: 'NoneType' object has no attribute 'lstrip' or TypeError: can only concatenate str (not "NoneType") to str.

Fix: treat None text as empty string at all three sites (title shape, body shape, notes frame).

RED

FAILED tests/test_pptx_none_text.py::test_pptx_converter_handles_none_shape_text
- PptxConverter threw AttributeError: 'NoneType' object has no attribute 'lstrip'
FAILED tests/test_pptx_none_text.py::test_pptx_converter_handles_none_notes_text
- PptxConverter threw TypeError: can only concatenate str (not "NoneType") to str

GREEN

tests/test_pptx_none_text.py::test_pptx_converter_handles_none_shape_text PASSED
tests/test_pptx_none_text.py::test_pptx_converter_handles_none_notes_text PASSED
2 passed in 0.65s

Full test_module_vectors.py (109 tests, covers real pptx conversion) still passes.