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 and fix that specific corner case.

```
EXCEPTION: (RuntimeException) preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=18554ea76ceb), phorge(head=diffAttach, ref.master=e11c5486c92b, ref.diffAttach=e11c5486c92b)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<arcanist>/src/error/PhutilErrorHandler.php:261]
#1 <#2> preg_match(string, NULL, NULL) called at [<phorge>/src/applications/differential/editor/DifferentialTransactionEditor.php:221]
```

Closes T15431

Test Plan: After applying this change on top of D25262, going to `/differential/diff/create/`, creating a diff, and entering nothing on the "Create Revision" page on `/differential/revision/attach/1/to/` correctly shows the errors "You must provide a test plan. Describe the actions you performed to verify the behavior of this change. Revisions must have a title." in web browser instead of an exception.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15431

Differential Revision: https://we.phorge.it/D25263