fix mysql incompatibility in tests

Author: lovasoa

tests/sql_test_files/data/coalesce_short_circuit.sql

@@ -4,4 +4,4 @@ from (select 'cached' as cached_response) cache_row;
 
 select '%20' as expected,
     coalesce(cached_response, sqlpage.url_encode(' ')) as actual
-from (select cast(null as varchar(16)) as cached_response) cache_row;
+from (select null as cached_response) cache_row;