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 ccfcaf3 (parseopt: values
of pathname type can be prefixed with :(optional), 2025-09-28) and
749d6d1 (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>

Author: benknoble

parse-options.c

@@ -226,7 +226,7 @@ static enum parse_opt_result do_get_value(struct parse_opt_ctx_t *p,
 		if (!value)
 			is_optional = 0;
 		value = fix_filename(p->prefix, value);
-		if (is_optional && is_empty_or_missing_file(value)) {
+		if (is_optional && is_missing_file(value)) {
 			free((char *)value);
 		} else {
 			FREE_AND_NULL(*(char **)opt->value);