Merge pull request #16106 from den-crane/test/bug_8580

test for the bug #8580

tests/queries/0_stateless/01518_cast_nullable_virtual_system_column.reference

+all tests passed

tests/queries/0_stateless/01518_cast_nullable_virtual_system_column.sql

+SELECT database FROM system.tables WHERE database LIKE '%' format Null;
+SELECT database AS db FROM system.tables WHERE db LIKE '%' format Null;
+SELECT CAST(database, 'String') AS db FROM system.tables WHERE db LIKE '%' format Null;
+SELECT CAST('a string', 'Nullable(String)') AS str WHERE str LIKE '%' format Null;
+SELECT CAST(database, 'Nullable(String)') AS ndb FROM system.tables WHERE ndb LIKE '%' format Null;
+SELECT 'all tests passed';
+