All usages of json_to_dict were replaced with the check_dict
validator. The check_dict validations can eventually be
extended to validate the keys and values of incoming data,
but now we just use check_dict([]) in all the places where
we had json_to_dict, which means we aren't checking for any
specific keys; we are just making sure it's a dictionary.
(imported from commit fc5add9a7ef149dfac2a9a6d9a153799c4c0c24d)