Required does not trigger validation when publishing with empty tag field.

Steps:

1. Create CT and add ibexa_taxonomy_entry_assignment field.
2. Set the field with "Required" set to true.
3. Save CT.
4. Go to Content structure.
5. Create item using this CT.
6. Leave tag field empty.
7. Fill other required fields.
8. Try to publish.

Actual result: Item is published with empty required tag field.
Expected result: Validation prevents publishing.