Handle case where Conduit repository.query doesn't send a value

Summary:
Fix the following error in NotifyNewCommitsToDiffusion:
count(): Parameter must be an array or an object that implements Countable

Fixes T1387.

Test Plan: Tested by NotifyNewCommitsToDiffusionTest::testHandle

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1387

Differential Revision: https://devcentral.nasqueron.org/D1539