parseopt: fix :(optional) at command line to only ignore missing files

parseopt: fix :(optional) at command line to only ignore missing files

Unlike the configuration option magic, the parseopt code also ignores
empty files: compare implementations from ccfcaf399f (parseopt: values
of pathname type can be prefixed with :(optional), 2025-09-28) and
749d6d166d (config: values of pathname type can be prefixed with
:(optional), 2025-09-28).

Unify the 2 by not ignoring empty files, which is less surprising and
the intended semantics from the first patch for config.

Suggested-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>