TMDM-7700: "Cannot use "IsEmptyOrNull' condition on a FK"

Fix incorrect query when "emptyOrNull(field)" is used instead of "or(isEmpty(field), isNull(field))" in query.
Amend existing unit test to ensure test coverage of UserQueryBuilder::emptyOrNull.
(https://jira.talendforge.org/browse/TMDM-7700)
Merge branch 'maintenance/5.6'