P3860R1: Address LWG feedback

[dalg24]

Addressing https://github.com/cplusplus/nbballot/issues/884#issuecomment-3487907524

Warning: I did not build locally; I got what I expect is an unrelated error.

Traceback (most recent call last):
  File "/Users/qdi/Projects/huixie90_cpp_papers/wg21/data/filters/wg21.py", line 577, in <module>
    pf.run_filters([
    ~~~~~~~~~~~~~~^^
        divspan,
        ^^^^^^^^
    ...<5 lines>...
        automatic_header_link,
        ^^^^^^^^^^^^^^^^^^^^^^
    ], prepare, finalize)
    ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/qdi/Projects/huixie90_cpp_papers/wg21/deps/python/lib/python3.13/site-packages/panflute/io.py", line 201, in run_filters
    prepare(doc)
    ~~~~~~~^^^^^
  File "/Users/qdi/Projects/huixie90_cpp_papers/wg21/data/filters/wg21.py", line 41, in prepare
    number = re.match("[PD]([0-9]+)R[0-9]+", document.upper())
                                             ^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'upper'
Error running filter /Users/qdi/Projects/huixie90_cpp_papers/wg21/data/filters/wg21.py:
Filter returned error status 1
make: *** [generated/concat.lwg.feedback.20230906.pdf] Error 83