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