title: How to deal with nulls in GraphQL schema
body: <p>I keep getting the "Cannot return null for non-nullable field Airline.id." error when FlightSchedule.operatingAirline is null (perfectly valid as per schema) and client queries for FlightSchedule.operatingAirline.id. How to fix this? Making Airline.id, Airline.code and Airline.name as nullable fixes this but is not the right way to solve this problem because if an Airline exist, these 3 fields will always exist too. Below is my schema:</p>
  
  <pre><code>type Airline {
      id: String!,
      code: String!,
      name: String!
  }
  
  type FlightSchedule {
      airline: Airline!
      operatingAirline: Airline
  }
  </code></pre>
  
  <p>And below is my query:</p>
  
  <pre><code>  getFlightSchedules {
      airline
      {
        id
        code
        name
      }
      operatingAirline
      {
        id
        code
        name
      }
    }
  </code></pre>