Fix PHP 8.1 "strlen(null)" exception in SVN commit query without identifier

Summary:
`strlen()` was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts `phutil_nonempty_string()` as a replacement.

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.

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/srv/phorge/phorge/src/applications/diffusion/query/DiffusionCommitQuery.php:779]
```

Closes T15936

Test Plan: Unclear.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15936

Differential Revision: https://we.phorge.it/D25821

src/applications/diffusion/query/DiffusionCommitQuery.php

@@ -776,7 +776,8 @@ final class DiffusionCommitQuery
               // See T3377.
               (int)$ref['identifier']);
           } else {
-            if (strlen($ref['identifier']) < $min_qualified) {
+            if (!phutil_nonempty_string($ref['identifier']) ||
+                strlen($ref['identifier']) < $min_qualified) {
               continue;
             }