Fix testUUIDField_pkAgregateCreate against MySQL

Constraint against testmodel_pk.uuid_field requires uuid_field being set.
```
ALTER TABLE `extensions_testagregatemodel` ADD CONSTRAINT `testmodel_pk_ptr_id_refs_uuid_field_91eb2fe4` FOREIGN KEY (`testmodel_pk_ptr_id`) REFERENCES `extensions_testmodel_pk` (`uuid_field`);
CREATE TABLE `extensions_testmanytomanymodel_many` (
    `id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY,
    `testmanytomanymodel_id` varchar(36) NOT NULL,
    `testmodel_field_id` integer NOT NULL,
    UNIQUE (`testmanytomanymodel_id`, `testmodel_field_id`)
);
```