Summary: Fixes T12090. In obscure situations lost to the mists of time, the `changes` column could be `null`. Force a string cast so the migration finishes, even though these changesets are likely meaningless.
Test Plan:
I did a force-reapply as a sanity check:
```
$ ./bin/storage upgrade -f --apply phabricator:20161213.diff.01.hunks.php
```
That went cleanly; it would only have caught dramatic errors, but I didn't completely butcher things.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T12090
Differential Revision: https://secure.phabricator.com/D17168
11f3a8cfca