PHP 8.5: Search: Do not pass an empty query key to isBuiltinQuery()
Summary:
Going to http://phorge.localhost/maniphest/query/advanced/, after `if ($this->queryKey == 'advanced') { $query_key = $request->getStr('query');` in `PhabricatorApplicationSearchController::processSearchRequest()`, `$query_key` remains null. Afterwards, `if ($engine->isBuiltinQuery($query_key))` calls PhabricatorApplicationSearchEngine::isBuiltinQuery($query_key) which tries `$builtins[$query_key]`.
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."
```
ERROR 8192: Using null as an array offset is deprecated, use an empty string instead at [/var/www/html/phorge/phorge/src/applications/search/engine/PhabricatorApplicationSearchEngine.php:757]
```
Closes T16361
Test Plan:
Go to http://phorge.localhost/maniphest/query/advanced/. Enter a term, run a query, save it.
Go to http://phorge.localhost/mail/query/advanced/ which triggers the issue.
Reviewers: O1 Blessed Committers, valerio.bozzolan, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16361
Differential Revision: https://we.phorge.it/D26530