Specify a null behavior for the callsign sort column.
Summary:
Fixes paging on the Diffusion Repository List.
PhabricatorRepositoryQuery needs to specify a behavior for `null` on the OderableColumns definition for the `callsign` column.
See https://phabricator.wikimedia.org/T180457
Test Plan:
1. On an instance with more than 100 repositories
* some of which are missing a callsign
2. Attempt to sort by callsign.
3. See the sorted results
Previously:
3. Exception: "Column "0" has null value, but does not specify a null behavior."
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D18773