Fix warning when optional attribute doesn't exist

When an attribute required is set to `false` and the attribute doesn't exist in the response, we get warning `undefined array key "attribute"` in the console, this update prevents that error

src/Appwrite/Utopia/Response.php

@@ -590,6 +590,10 @@ class Response extends SwooleResponse
                 }
             }
 
+            if(!$data->isSet($key) && !$rule['required']) { // skip attribute in response if not required and values does not exist
+                continue;
+            }
+            
             if ($rule['array']) {
                 if (!is_array($data[$key])) {
                     throw new Exception($key . ' must be an array of type ' . $rule['type']);