Fix PHP 8.1 "strlen(null)" exception listing >100 task search results

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.

```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter freebsd#1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=df6c315ace5f), phorge(head=master, ref.master=7040bd525764)
  #0 <freebsd#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/maniphest/query/ManiphestTaskQuery.php:1039]
```

Closes T15604

Test Plan: Have more than 100 tasks, run a broad search with more than 100 results, try to go to next page of results.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15604

Differential Revision: https://we.phorge.it/D25392

Author: aklapper

src/applications/maniphest/query/ManiphestTaskQuery.php

@@ -1036,7 +1036,7 @@ private function parseCursor($cursor) {
       $parts[] = null;
     }
 
-    if (!strlen($parts[1])) {
+    if (!phutil_nonempty_string($parts[1])) {
       $parts[1] = null;
     }