PHP 8.5: Fix an invalid null array key in PhabricatorSearchOrderField
Summary:
Setting null as an array key is deprecated since PHP 8.5 per https://www.php.net/releases/8.5/en.php: "Using null as an array offset or when calling array_key_exists() is now deprecated. Use an empty string instead."
Do not try to look up an `orderAlias` if `ValueForControl` in parent `PhabricatorSearchField` is null anyway (`$value` is only set if `readValueFromSavedQuery()` gets called as that sets `$this->value = $this->didReadValueFromSavedQuery($value);`).
```
ERROR 8192: Using null as an array offset is deprecated, use an empty string instead at [/var/www/html/phorge/phorge/src/applications/search/field/PhabricatorSearchOrderField.php:39]
```
Closes T16358
Test Plan: Log in and go to http://phorge.localhost/maniphest/
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16358
Differential Revision: https://we.phorge.it/D26527