Correctly handle case where no ssh keys are found

Summary: If no ssh keys are in phabricator, bin/ssh-auth errors with undefined `$lines`. This fixes that case and explicitly tells the user no rows were found.

Test Plan: Ran bin/ssh-auth before and after change with no ssh keys in the system. Error goes away after change.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: epriestley

CC: epriestley, aran, Korvin

Differential Revision: https://secure.phabricator.com/D7675