Force 'changes' to a string in hunk migration

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