Fix LaTeX builder crash on tables with no body rows

[veeceey]

Summary

Fixes #14271.

LaTeXFootnoteVisitor.depart_table() unconditionally calls next(node.findall(nodes.tbody)), which raises StopIteration when a table has header rows but no body rows. This happens when external parsers like myst_parser produce a docutils table node tree from Markdown tables that have only a header row (e.g. | Key | P | Summary |\n| --- | --- | --- |).

The fix:

• Use next(..., None) to safely handle the missing tbody
• When tbody is present, behavior is unchanged (footnotes are inserted at the head of tbody)
• When tbody is absent, any collected table footnotes are placed after the table node instead, preserving footnote rendering

Test:

• Added test_latex_table_empty_body which constructs a table node with thead but no tbody (mirroring what myst_parser produces) and verifies that LaTeXFootnoteVisitor processes it without raising StopIteration
• Verified the test fails with StopIteration on the unfixed code and passes with the fix
• All existing LaTeX builder tests continue to pass

Test plan

• New unit test test_latex_table_empty_body passes
• All existing LaTeX table tests pass (9/9)
• All existing LaTeX footnote tests pass (5/5)
• Full LaTeX test suite passes (82 passed, 11 skipped for missing LaTeX styles)
• ruff check passes on both modified files

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.