Dashboards - fix optionality of SearchEngines

Summary: Ref T7234. I didn't know about this spot in D11750.

Test Plan: ..the next diff really makes this work for the T7234 scenario.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7234

Differential Revision: https://secure.phabricator.com/D11751