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]
```
Closes T15911