R1:2d997c3f107e

Avoid "Undefined index" error in project.search API when project_customfieldstorage table has additional fields not requested

Summary:
Do not trigger an "Undefined index" error in PhabricatorCustomFieldStorage when calling the `project.search` Conduit API without passing search parameters when the local project_customfieldstorage table has additional fields configured not requested in the API call.

Closes T15688

Test Plan: See above; `curl -k -s -X GET http://phorge.localhost/api/project.search?api.