Fix a minor issue in Nuance which could cause the trigger daemon to poll too often

Summary: Ref T10537. Currently, when you have at least two cursors, the daemon can poll too frequently when processing the last source because it never hits the end-of-list condition.

Test Plan:

• Ran bin/phd debug trigger.
• Observed huge volumes of output before change as triggers fired as fast as possible.
• Observed reasonable poll frequency after change.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10537

Differential Revision: https://secure.phabricator.com/D15464