Fix panic when mutations or subscriptions is not configured

[linniksa]

when call

mutation { a }

on schema that mutations is not configured, i got:

{
  "data": null,
  "errors": [
    {
      "message": "runtime error: invalid memory address or nil pointer dereference",
      "locations": []
    }
  ]
}

this PR fix that

[coveralls]

Coverage increased (+0.4%) to 92.237% when pulling 90fefa5 on linniksa:fix-unsupported-operation-error into 21fe71d on graphql-go:master.

[chris-ramon]

LGTM 👍 It aligns with the graphql-js reference implementation. — Thanks a lot @linniksa!