fix(pptx): handle missing text frames

[MukundaKatta]

Summary

• treat shape.text and notes_frame.text as empty strings when python-pptx returns None
• avoid failing the whole PPTX conversion on malformed or third-party-generated decks
• add a regression test that patches a PPTX text frame getter to return None

Verification

• PYTHONPYCACHEPREFIX=/tmp/pycache-markitdown python3 -m py_compile packages/markitdown/src/markitdown/converters/_pptx_converter.py packages/markitdown/tests/test_module_misc.py
• targeted pytest collection was blocked locally because this environment does not have onnxruntime, which is imported through magika during test startup

Closes #1808