GraphQL undefined index error

Problem/Motivation

When executing a graphql-query which queries for a redirect for a given path, which doesn't have a redirect, an "Undefined index" error is shown in the log.

Steps to reproduce

1. Open GraphQL Explorer
2. Execute a query for a redirect on a path which does not have a redirect, like this:
   

   query RedirectIndexUndefined {
     redirect(path:"/"){
     	url
     }
   }

3. Notice "Undefined index"...

Proposed resolution

On the ResolverHelperTrait, add a check if the array key exists in the addSimpleCallbackFields function.