streamId property Null safety

Field description: ID of the live stream output to CDN If this parameter is not set, the SDK will execute the default logic, that is, it will mix the multiple streams in the room into the video stream of the caller of the API, i.e., A + B => A; If this parameter is set, the SDK will mix the multiple streams in the room into the live stream ID you specify, i.e., A + B => C. Recommended value: default value: null, that is, the multiple streams in the room will be mixed into the video stream of the caller of this API.