Fix PhabricatorBot macro cacheing

Summary:
Previously, if there were no macros, we would ping conduit for a list of macros until we got something. Now we cache false when there are no results.
T3045

Test Plan: Ensure the init doesn't call the macro.query conduit method more than once during the PhabricatorBot's lifetime.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Maniphest Tasks: T3045

Differential Revision: https://secure.phabricator.com/D6671