Drupal 7 + Services : Error 500 on user registration (Integrity constraint violation: 1048 Column 'uid' cannot be null)
I think that on the other side of your code you are trying to save the user. So, to create a new user using user_save() function you have to left the first parameter blank so a new user is created. Also, add the parameter true to your json_decode($response, true) to obtain an array rather than an object.

$account = user_save('', $fields);