Fixes broken search if no task ids created by D2771.

Summary: I forgot to handle the case where there was no task ids entered in D2771, but this corrects that issue.

Test Plan: Search with no task ids...

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1365

Differential Revision: https://secure.phabricator.com/D2773