Hasura is not checking response from action handler. Field in schema marked as non nullable, e.g.:

type SampleOutput {
  accessToken : String!
  userId : Int!
}

and badly implemented handler which doesn't return mandatory fields cause response:

{
  "data": {
    "actionName": {
      "accessToken": null,
      "userId": null
    }
  }
}

Schema should be accepted and thus bad answer from resolver can be ignored. In this way works other GraphQL servers like Apollo or GraphQL JS.