Fix PHP 8.1 "strlen(null)" exception which blocks task Bulk Editor page

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.

Closes T15391

Test Plan:
Applied this change and afterwards, "Bulk Editor" page on `/maniphest/bulk/` rendered correctly.

To test that:

- visit a list of some Tasks (like `/maniphest/`)
- filter something (this can be done in multiple ways)
- select Tasks
- click on Bulk Edit Selected Tasks

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15391

Differential Revision: https://we.phorge.it/D25223

src/applications/transactions/bulk/PhabricatorBulkEngine.php

@@ -153,7 +153,7 @@ abstract class PhabricatorBulkEngine extends Phobject {
       ->setViewer($viewer);
 
     $query_key = $request->getURIData('queryKey');
-    if (strlen($query_key)) {
+    if (phutil_nonempty_string($query_key)) {
       if ($search_engine->isBuiltinQuery($query_key)) {
         $saved = $search_engine->buildSavedQueryFromBuiltin($query_key);
       } else {