Fix PHP 8.1 "preg_match(null)" exception when cloning a repository with no URI set

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=arcpatch-D25250, ref.master=18554ea76ceb, ref.arcpatch-D25250=fade4603a799), phorge(head=diffusionEmptyBranchHistory, ref.master=e11c5486c92b, ref.diffusionEmptyBranchHistory=76f042e4b969)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<arcanist>/src/error/PhutilErrorHandler.php:261]
  #1 <#2> preg_match(string, NULL) called at [<phorge>/src/applications/diffusion/request/DiffusionGitRequest.php:6]
```

Closes T15435

Test Plan: After applying this change, try to "Clone" a Repository with no URI set shows the overlay dialog `Clone Repository - Repository has no URIs set.` as expected.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15435

Differential Revision: https://we.phorge.it/D25266

src/applications/diffusion/request/DiffusionGitRequest.php

@@ -3,6 +3,9 @@
 final class DiffusionGitRequest extends DiffusionRequest {
 
   protected function isStableCommit($symbol) {
+    if ($symbol === null) {
+      return false;
+    }
     return preg_match('/^[a-f0-9]{40}\z/', $symbol);
   }