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.

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.