Don't treat configured but absent engine as executable

E.g. if you have $wgExternalDiffEngine = 'wikidiff2' but after
a PHP update you no longer have the module you still shouldn't attempt
to shell out to some nonexistent wikidiff2.

Bug: T74030
Change-Id: I745cd1cb2e152f4fbb95c8f782d70117f8c844f1