skip tests that require ingores_nulls_in_unique_constraints feature
skip test_related_null_to_field it requires ignores_nulls_in_unique_constraints feature
(cherry picked from commit 5aee09b3be381f6ec33290f9ed90996fa0624609)
fix test backends.FkConstraintsTests.test_check_constraints multiple_database.RouterTestCase.test_generic_key_cross_database_protection