Introspecting __typename field panics the service

[MohamedSabthar]

Description:
Introspection with the following GraphQL queries causes the service to crash with a {ballerina/lang.map}KeyNotFound error:

{
  __type(name: "Profile") {
    __typename
  }
}

{
  __schema {
    types {
      name
      __typename
    }
  }
}

Steps to reproduce:
Run the queries against the example at https://ballerina.io/learn/by-example/graphql-returning-record-values/.

Expected behaviour: introspection queries should return "__Type" as the value for __typename

Affected Versions:

OS, DB, other environment details and versions:

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