Enum generation: OpenAPI 3.1 null enum value doesn't generate enum

[DanDeMicco]

Description

A brief description of the bug.

I have a nullable enum yaml file (Open API 3.1) defined like:

type:
  - string
  - 'null'
...
enum:
  - VALUE1
  - VALUE2
  - null

which i have in my schema as

values:
  $ref: <relative_path>/values.yaml

This will output in the TS schema output:

values: "VALUE1" | "VALUE2" | null

with no enum emitted

Name	Version
openapi-typescript	7.3.0
Node.js	v18.20.4
OS + version	macOS 14.6.1

Reproduction

How can this be reproduced / when did the error occur?

1. create a nullable enum similar to the above example, reference it in API

npx openapi-typescript ./openapi.yaml -o ./src/typings/schema.d.ts --enum --alphabetize --dedupe-enums

3. check output schema.d.ts

Expected result

1. It should output the enum for valid values (e.g. export enum Values ...
2. the output in the TS schema should be values: Values | null

Checklist

• My OpenAPI schema passes the Redocly validator (npx @redocly/cli@latest lint)
• I’m willing to open a PR (see CONTRIBUTING.md)