R1:44a8a1c408ed

Fix PHP 8.1 "preg_match(null)" exception on "Create Revision" page in Differential when not entering data

Summary:
`preg_match()` does not accept passing null as the `$subject` string parameter in PHP 8.1.

Thus add a `phutil_nonempty_string()` check if the `$subject` parameter is a non-empty string.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate…