The iif operation does not work with literal tables in the Snowflake backend

[radeusgd]

See the test

+
+            # Literal tables only make sense in the DB.
+            if setup.is_database then group_builder.specify "iif should work with literal tables" <|
+                literal_table = setup.light_table_builder [["X", [True, False, Nothing]], ["row_id", [1, 2, 3]]] . sort "row_id"
+                literal_table.at "X" . value_type . should_equal Value_Type.Boolean
+                c = literal_table.at "X" . iif 10 20
+                c.to_vector . should_equal [10, 20, Nothing]

Currently it fails on Snowflake.