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 #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]
#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