fix: remove parent_type.fields check in analyzer

[al-yanna]

Context

Follow-up to #44

For direct fields in Union types, for example, __typename in

query {
  searchResult(query: $query) {
    __typename
    ... on Project {
      id
    }
  }
}

the parent_type.fields[node.name] check results in

undefined method `fields' for class [UnionType] (NoMethodError)

Changes

• Update parent_type.fields check to be node&.respond_to?(:name)

Testing

• Unit test (Analyzer)

[aryascripts]

Small comment, but this is looking good!