pathchk: reject an empty file name in portability modes to match GNU

[AlejandroCoronadoN]

In pathchk's portability modes (-p, -P, --portability), an empty file name diverges from GNU:

• -P prints pathchk: '': No such file or directory (it runs the filesystem check first and never reaches the empty-name check).
• -p and --portability print empty file name without the pathchk: prefix.

GNU rejects an empty file name in any portability mode before touching the filesystem, always as pathchk: empty file name:

$ pathchk -P ''             # pathchk: '': No such file or directory   (GNU: pathchk: empty file name)
$ pathchk -p ''             # empty file name                          (GNU: pathchk: empty file name)
$ pathchk --portability ''  # empty file name                          (GNU: pathchk: empty file name)

This adds the early empty-name check (using the existing localized string) for all portability modes. Default mode (pathchk '') is unchanged.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/misc/tty-eof (fails in this run but passes in the 'main' branch)

[cakebaker]

This change makes the "empty file name" checks in check_basic and check_extra unreachable if an empty file name is provided and so you can remove them.

[cakebaker]

You can remove this test as it tests the same as your test, though your test is better.