Fix PHP 8.1 "class_exists(null)" exception on PhabricatorStandardSelectCustomFieldDatasource
Summary:
Throw a less cryptic exception when no (required) class parameter is passed to `PhabricatorStandardSelectCustomFieldDatasource`.
```
ERROR 8192: class_exists(): Passing null to parameter #1 ($class) of type string is deprecated at [/var/www/html/phorge/phorge/src/infrastructure/customfield/datasource/PhabricatorStandardSelectCustomFieldDatasource.php:22]
EXCEPTION: (Exception) Custom field class "" does not exist. at [<phorge>/src/infrastructure/customfield/datasource/PhabricatorStandardSelectCustomFieldDatasource.php:23]
```
Closes T16298
Test Plan:
* Go to http://phorge.localhost/typeahead/class/?class=PhabricatorStandardSelectCustomFieldDatasource
* Get a more helpful exception `"You must set a custom field class!"`
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16298
Differential Revision: https://we.phorge.it/D26423