Fix success response nullable VK_T_NO parameter

src/Messages/CompleteRequest.php

@@ -100,7 +100,7 @@ protected function validateResponseParameters()
 
         //check for missing fields, will throw exc. on missing fields
         foreach ($responseFields as $fieldName => $usedInHash) {
-            if (! isset($response[$fieldName])) {
+            if (! array_key_exists($fieldName, $response)) {
                 throw new InvalidRequestException("The $fieldName parameter is required");
             }
         }