Tighten Ferret query parsing of empty tokens and empty functions

Summary:
Ref T13509. Certain query tokens like title:="" are currently accepted by the parser but discarded, and have no impact on the query. This isn't desirable.

Instead, require that tokens making an assertion about field content must be nonempty.

Test Plan: Added unit tests, made them pass.

Maniphest Tasks: T13509

Differential Revision: https://secure.phabricator.com/D21106