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

src/applications/repository/query/PhabricatorRepositoryQuery.php

@@ -425,6 +425,7 @@ final class PhabricatorRepositoryQuery
         'type' => 'string',
         'unique' => true,
         'reverse' => true,
+        'null' => 'tail',
       ),
       'name' => array(
         'table' => 'r',