Fix fatal during redirection safety check for searching for Phabricator base-uri with no trailing slash

Summary: Fixes T13412. If you search for "https://phabricator.example.com" with no trailing slash, we currently redirect you to "", which is fouled by a safety check in the redirection flow.

Test Plan:

• Searched for "https://local.phacility.com"; before: fatal in redirection; after: clean redirect.
• Searched for other self-URIs, got normal redirects.

Maniphest Tasks: T13412

Differential Revision: https://secure.phabricator.com/D20794