Set config argument default value to null

src/Datasource/ConnectionManager.php

@@ -83,7 +83,7 @@ class ConnectionManager {
  * @param array $config An array with a `url` key mapping to a string dsn
  * @return mixed null when adding configuration and an array of configuration data when reading.
  */
-	public static function parseDsn($config) {
+	public static function parseDsn($config = null) {
 		$config = static::_parseDsn($config);
 
 		if (isset($config['user'])) {