Ensure migration 209 works with NULL fkey values

Modify the sub-query on migration 209's backup to not select NULL
items which cause the NOT IN to always be false and consequently
the backup not to run resulting in a violation of the foreign keys.

To test, add an instance with NULL uuid to check the backups still
work correctly and avoid violating the foreign key constraint.
If there are NULL values in a NOT IN () set the result is always
false. Therefore having this instance inserted causes migration 209
to fail unless the IN set sub-query is modified appropriately.

Change-Id: Id0f8c8cb082df8861e85f925b979ddd934a95bfc
Closes-Bug: 1240325
Merge "Ensure migration 209 works with NULL fkey values"