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.token=api-xxxxxxxxxxxx | jq -r for comparison

Reviewers: O1 Blessed Committers, 20after4

Reviewed By: O1 Blessed Committers, 20after4

Subscribers: 20after4, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15688

Differential Revision: https://we.phorge.it/D25493