fix(docx): resolve math converter crash on empty runs in OMML to LaTeX

[saitejabandaru-in]

Summary

Resolves #2188.

Root Cause

In omml.py, the do_r function retrieves text from a math run element via elm.findtext("./{0}t".format(OMML_NS)). If a math run (<m:r>) has formatting properties (<m:rPr/>) but no <m:t> text child (which some Word editors or copy-pasted equations produce), findtext() returns None.
Attempting to iterate over None (for s in None) raises TypeError: 'NoneType' object is not iterable.

Because math preprocessing in pre_process_docx() runs on the entire document.xml file in a single try-except block, a single crash in do_r silently drops all equations in the document, falling back to original unconverted XML.

Changes

• Guard against None return values in do_r() before iteration.
• Add a unit test (test_docx_empty_math_run) in test_module_misc.py using a mock XML structure to verify the fix works and prevents regressions.