[COOK-4227] bugfix mysql syntax privileges_exist

Small change to fix the mysql syntax error:
ERROR 1064 (42000) at line 1: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'for 'username'@'127.0.0.1'@'%'' at line 1
The error means the only_if in execute "Grant WordPress MySQL Privileges" will always be empty, so the execute will always run.