Fix file-name-case-insensitive-p in ffap (bug#56443)

Fix file-name-case-insensitive-p in ffap (bug#56443)

Don't crash if the file name argument to file-name-case-insensitive-p,
after expansion, doesn't have a parent directory.  This occurs
when calling ffap on something that looks like an email address.

* src/fileio.c (Ffile_name_case_insensitive_p): Return nil if no file
or parent directory could be found.
* test/src/fileio-tests.el (fileio-tests--identity-expand-handler)
(fileio--file-name-case-insensitive-p): New test.