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