GraphQL __typename introspection is crashed when fields return a nullable, union type array

[DimuthuMadushan]

Description:
If a GraphQL resource function includes a nullable, union type array as the return type, the __typename for that field can not be resolved properly.
Consider the following GrpahQL service:

import ballerina/graphql;

type Character Human|Droid;

distinct isolated service class Human {
    resource function get name () returns string {
        return "human-1";
    }
}

distinct isolated service class Droid {
    resource function get id () returns string {
        return "droid-1";
    }
}
service /graphql on new graphql:Listener(9090) {
    resource function get character() returns Character?[] {
        return [new Human(), new Droid(), ()];
    }
}

if we query this as follow:

{
    character {
        __typename
    }
}

the result should be:

{
  "data": {
    "character": [
      {
        "__typename": "Human"
      },
      {
        "__typename": "Droid"
      },
      null
    ]
  }
}

Currently, this service is crashed when querying.