R1:a1f12b4ac7af

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…