INT-19075: PHPUnit 8.1 fix Passing null to parameter of type X is deprecation

Author: danielcifuentesopen

classes/lock/redis_lock_factory.php

@@ -424,6 +424,6 @@ protected function get_lock_value() {
         return http_build_query(array(
             'hostname' => $this->get_hostname(),
             'processid' => getmypid(),
-        ), null, '&');
+        ), '', '&');
     }
 }