[Bug]: Nullable Enums produce oneOf

[rima1098]

Describe the bug

When declaring models with Enums that are nullable the generated OpenAPI 3.0 description does produce a oneOf descriminator. (see playground link).

Instead of receiving

enumProp:
    oneOf:
        - $ref: '#/components/schemas/MyEnum'
    nullable: true

i would expect to receive:

enumProp:
    allOf:
        - $ref: '#/components/schemas/MyEnum'
    nullable: true

Reproduction

https://typespec.io/playground?c=aW1wb3J0ICJAdHlwZXNwZWMvaHR0cCI7DQrSGnJlc3TWGm9wZW5hcGkzxB4NCnVzaW5nIFR5cGVTcGVjLlJlc3Q70RZIdHRwxS5lbnVtIE15RcQHew0KICAgIHZhbHVlMTvLDTLMDTM7DQp9xDttb2RlbCBSZXF1aXJlZE5vdE51bGxhYmxlyEnEXFByb3A6x2HXOd82IHwgbnVsbM49T3B0aW9uYWzbdj%2FIQdY53zbId8U9&e=%40typespec%2Fopenapi3&options=%7B%7D

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.

[timotheeguerin]

Yeah seems like a bug for other types with $ref is does make an allOf
Playground