TMDM-7530: "Display warning during validation when a mandatory field is not visible to all users"

Fix incorrect warning reported when a non-mandatory element has a visibility rule *and* is at first level in entity.
Adds unit test for testing data model where such condition is met.
(https://jira.talendforge.org/browse/TMDM-7530)
Merge branch 'maintenance/5.6'