fix(docx): an equation without its properties child drops every equation

[kevin (kevin9327)]

Converting a .docx whose equations were written by something other than Word produces the document's prose with every equation missing — no error, no warning, just no math (issue #1512).

Every OMML element's properties child is optional. <m:f> without <m:fPr> is a fraction with default properties, and the same goes for <m:d>, <m:acc>, <m:bar> and <m:groupChr>. Word always writes the child, because it carries the control run's formatting — in tests/test_files/equations.docx every <m:fPr> holds nothing but a <m:ctrlPr> — but nothing else has to.

do_f, do_d, do_acc, do_bar and do_groupchr indexed the child directly (c_dict["fPr"]), so an equation that omits it raises KeyError. The five already carry the spec's defaults; they just crash before reaching them.

The damage is not local. _pre_process_math runs over all of word/document.xml inside a blanket except Exception, so one such equation discards the OMML rewrite for the whole part, and Mammoth cannot render raw OMML. Taking the existing fixture and dropping the element a non-Word producer would not have written:

equations.docx                 : 4 lines containing LaTeX
equations.docx without <m:fPr>: 0 lines containing LaTeX
equations.docx without <m:dPr>: 0 lines containing LaTeX

After the fix, 4 and 4 and 4.

This is the same failure class as #2368, which gives do_nary the spec default for an omitted m:chr; these five crash a step earlier, on the properties element itself. The fix reads the child through a helper that substitutes an empty one, so every property falls through to the default it already has.

Reproduction

New test file, on unmodified main (packages/markitdown):

$ python -m pytest tests/test_docx_math_optional_properties.py -q
FAILED tests/test_docx_math_optional_properties.py::test_omitted_properties_convert_like_default_properties[f]
FAILED tests/test_docx_math_optional_properties.py::test_omitted_properties_convert_like_default_properties[d]
FAILED tests/test_docx_math_optional_properties.py::test_omitted_properties_convert_like_default_properties[acc]
FAILED tests/test_docx_math_optional_properties.py::test_omitted_properties_convert_like_default_properties[bar]
FAILED tests/test_docx_math_optional_properties.py::test_omitted_properties_convert_like_default_properties[groupChr]
FAILED tests/test_docx_math_optional_properties.py::test_fraction_without_properties_still_converts
FAILED tests/test_docx_math_optional_properties.py::test_delimiter_without_properties_uses_the_default_parentheses
FAILED tests/test_docx_math_optional_properties.py::test_document_keeps_its_equations_without_the_properties_child[fPr]
FAILED tests/test_docx_math_optional_properties.py::test_document_keeps_its_equations_without_the_properties_child[dPr]
9 failed, 1 passed in 2.86s

The unit failures are KeyError: 'fPr' and friends; the end-to-end one is the whole document going dark:

>       assert _latex_line_count(_docx_without(element_name)) == expected
E       AssertionError: assert 0 == 4

The one test that passes before and after is the pin: <m:dPr> with an explicit begChr/endChr still converts to \left[x\right] rather than the default parentheses.

With the fix:

$ python -m pytest tests/test_docx_math_optional_properties.py -q
10 passed in 4.10s

Verification

No fixture is modified: the end-to-end test rebuilds equations.docx in memory.

$ python -m pytest tests/test_docx_math.py tests/test_docx_math_accents.py \
    tests/test_docx_math_symbols.py tests/test_docx_omml.py \
    tests/test_module_vectors.py -q
125 passed in 45.09s   # origin/main
125 passed in 38.38s   # this branch

black reports both touched files unchanged.