Add missing tests for NOT IN accepting empty values and refactor

Go back to operators map since the previously defined
Operator constructors added innecesary complexity also
rename Path to EmbedPath

test/fixtures/data.sql

@@ -294,6 +294,9 @@ INSERT INTO w_or_wo_comma_names VALUES ('Smith, Joseph');
 INSERT INTO w_or_wo_comma_names VALUES ('David White');
 INSERT INTO w_or_wo_comma_names VALUES ('Larry Thompson');
 
+TRUNCATE TABLE items_with_different_col_types CASCADE;
+INSERT INTO items_with_different_col_types VALUES (1, null, null, null, null, null, null, null);
+
 --
 -- PostgreSQL database dump complete
 --