openapi to tsp converter doesn't like unions

[timotheeguerin]

openapi: 3.0.0
info:
  title: (title)
  version: 0.0.0
tags: []
paths: {}
components:
  schemas:
    Abc:
      type: object
      required:
        - name
      properties:
        name:
          type: string
    MyUnion:
      oneOf:
        - type: string
        - $ref: '#/components/schemas/Abc'

converts to

import "@typespec/http";
import "@typespec/openapi";
import "@typespec/openapi3";

using Http;
using OpenAPI;

@service({
  title: "(title)",
})
@info({
  version: "0.0.0",
})
namespace title;

model Abc {
  name: string;
}

@oneOf
model MyUnion {}