Missing generation of union types (typescript-type-graphql)

Describe the bug typescript-type-graphql plugin does not generate the necessary createUnionType function when generating a union type.

To Reproduce Steps to reproduce the behavior: Use a union type as a property of a document and generate using the typescript-type-graphql plugin.

https://codesandbox.io/s/strange-northcutt-ki24s?file=/types.ts

1. My GraphQL schema:

type ALevel {
	subject: ALevelSubject!
	grade: ALevelGrade!
}

type BTEC {
	subject: String!
	grade: BTECGrade!
}

union Qualification = ALevel | BTEC

type Information {
	qualifications: [Qualification!]!
}

3. My codegen.yml config file:

overwrite: true
schema: './src/*.gql'
generates:
../graphql/src/qlModels.ts:
    plugins:
      - typescript-type-graphql
    config:
      namingConvention: keep

Expected behavior a createUnionType function is generated for each union type.

Environment:

• OS: Ubuntu 20.04
• @graphql-codegen/cli: 1.19.4
• @graphql-codegen/typescript-type-graphql: 1.18.2
• NodeJS: 14.15.1