10999_update_principaluser migration fails in certain circumstances

[bferguso]

If upgrading an existing database that has resources created by a user that no longer exists, the migration fails due to a foreign key constraint violation (resource_instances.principle_user -> auth_user.id).

The second SQL statement should have a clause that excludes out any resources that were created by users that don't exist in the auth_user table.

NB - the fix requires an update to an existing migration rather than adding an additional migration due to the nature of the problem.