Fix omml template bugs.

[afourney]

Fix fallback handling for unrecognized OMML attribute values

get_val() returned the lookup key itself when an OMML attribute value was not
found in its mapping table. Callers use that result as a str.format() template,
so an unrecognized value produced malformed LaTeX or raised (ValueError,
IndexError, KeyError) rather than falling back to a sensible default. Because
pre_process_docx catches exceptions per part, a single unrecognized value
silently disabled equation conversion for an entire document.xml.

get_val() now falls back to the caller's default on a miss, so only templates
defined in latex_dict.py reach .format(). The two call sites that legitimately
pass a character through as literal text (do_d delimiters, do_nary operators)
now use a new get_char() helper that retains the previous behavior.

do_groupchr() passed no default at all; added CHR_DEFAULT["GROUP_CHR_VAL"]
(\underbrace, matching ECMA-376's U+23DF default for the grouping character).
This also fixes an AttributeError when <m:groupChr> has no <m:chr> child.

Affected handlers: do_acc, do_bar, do_f, do_groupchr. Recognized values are
unchanged. Existing tests pass, including test_docx_equations.