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

resources/sql/autopatches/20161213.diff.01.hunks.php

@@ -28,7 +28,8 @@ foreach (new LiskRawMigrationIterator($conn, $src_table) as $row) {
     DifferentialModernHunk::DATATYPE_TEXT,
     'utf8',
     DifferentialModernHunk::DATAFORMAT_RAW,
-    $row['changes'],
+    // In rare cases, this could be NULL. See T12090.
+    (string)$row['changes'],
     $row['dateCreated'],
     $row['dateModified']);
 }