[migration tsp] How to support optional response body

[v-jiaodi]

1.Spec PR: Azure/azure-rest-api-specs#39837
response:

"responses": {
  "200": {
    "description": "OK"
  },
  "201": {
    "description": "Partner Topic update request accepted.",
    "schema": {
      "$ref": "#/definitions/PartnerTopic"
    }
  },

• typespec link: https://github.com/welovej/azure-rest-api-specs/blob/52d7e2a38508770e1a686349d06aba81eb35fe28/specification/eventgrid/resource-manager/Microsoft.EventGrid/EventGrid/PartnerTopic.tsp#L53

• original swagger link: https://github.com/Azure/azure-rest-api-specs/blob/main/specification/eventgrid/resource-manager/Microsoft.EventGrid/EventGrid/preview/2025-07-15-preview/EventGrid.json#L8855

• generated swagger link: https://github.com/welovej/azure-rest-api-specs/blob/52d7e2a38508770e1a686349d06aba81eb35fe28/specification/eventgrid/resource-manager/Microsoft.EventGrid/EventGrid/preview/2025-07-15-preview/EventGrid.json#L8867

• hlc link in sdk repo: https://github.com/Azure/azure-sdk-for-js/blob/main/sdk/eventgrid/arm-eventgrid/src/operations/partnerTopics.ts#L306

• typespec code:

update is ArmCustomPatchSync<
    Employee,
    Azure.ResourceManager.Foundations.ResourceUpdateModel<Employee, EmployeeProperties>,
    Response = OkResponse | ArmResourceCreatedSyncResponse<Employee>
  >;

• generated swagger:

responses": {
          "200": {
            "description": "The request has succeeded."
          },
          "201": {
            "description": "Resource 'Employee' create operation succeeded",
            "schema": {
              "$ref": "#/definitions/Employee"
            }
          },
          "default": {
...
        }

in this case, tcgc will use the response type from 201 as the method.response

here is the playground

this issue is to confirm how to support optional response body