Migrate image field uuid

Problem/Motivation

MigrateFieldTest.php tests the migrated uuid. However, the uuid doesn't seem to be migrated, it is a NULL, not an empty string.

$this->assertEqual($settings['default_image']['uuid'], '');

Proposed resolution

None yet

Remaining tasks

Decide if the uuid is to be migrated or not.
Modify and document MigrateFieldTest.php based on that decision.

User interface changes

None

API changes

None