title: GraphQL: prevent empty array as input
body: <p>I'm learning GraphQL now and I faced a problem that I am unable to resolve.</p>
  
  <p>i have defined type in schema as</p>
  
  <p>input School {
      name: String!
      exams: [subject!]!
  }
  input subject {
      name: String!
      mark: Number!
  }</p>
  
  <p>But while running the server.
  I give data as
  "data":
  {
  "name":"varun",
  "exams":[],
  }</p>
  
  <p>Server does not throw any validation error.
  Please suggest me solution.
  Is there any possible way to get validation error on passing exams as empty array.</p>
  
  <p>I have tried alternate methods, but nothing works on schema level. I have to put a check explicitly to check exam field should not be an empty array.
  Thanks</p>