Fix PHP 8.1 "str_replace(null)" exception in DivinerAtomRef

Summary:
Passing null instead of a string or array to `str_replace()` deprecated since PHP 8.1.

Thus do not create a title array with a `null` entry in `DivinerFindController` when there is no `$query_text`, later to be read via `$this->titles` in `DivinerAtomRef`.

```
ERROR 8192: str_replace(): Passing null to parameter freebsd#3 ($subject) of type array|string is deprecated at [/var/www/html/phorge/phorge/src/applications/diviner/atom/DivinerAtomRef.php:205]
  #0 str_replace(string, string, NULL) called at [<phorge>/src/applications/diviner/atom/DivinerAtomRef.php:205]
  freebsd#1 DivinerAtomRef::normalizeTitleString(NULL) called at [<phorge>/src/applications/diviner/query/DivinerAtomQuery.php:344]
```

Credits to valerio.bozzolan for finding the right spot in the code.

Closes T15911

Test Plan: Go to http://phorge.localhost/diviner/find/ (not passing a `name` URI parameter), optionally with D25768 applied to avoid another exception

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15911

Differential Revision: https://we.phorge.it/D25769

Author: aklapper

src/applications/diviner/controller/DivinerFindController.php

@@ -58,7 +58,9 @@ public function handleRequest(AphrontRequest $request) {
 
     if (!$atoms) {
       $title_query = clone $query;
-      $title_query->withTitles(array($query_text));
+      if (phutil_nonempty_string($query_text)) {
+        $title_query->withTitles(array($query_text));
+      }
       $atoms = $title_query->execute();
     }