fix: type error

PHP Deprecated: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated

Author: kenjis

system/API/ResponseTrait.php

@@ -358,7 +358,7 @@ protected function format($data = null)
      */
     protected function setResponseFormat(?string $format = null)
     {
-        $this->format = strtolower($format);
+        $this->format = ($format === null) ? null : strtolower($format);
 
         return $this;
     }