Make RunObject.expires_at nullable

[davidmigloz]

RunObject.expires_at can be null (e.g. when the run is completed).

Example:

{
  "id": "run_soD0ciq9WNNZ2FL6zDlcur7b",
  "object": "thread.run",
  "created_at": 1703355405,
  "assistant_id": "asst_uzsXn9NW5FURQq4qU8Byqr0T",
  "thread_id": "thread_rhvsVlbAf00zQ9DgMTfZE7BS",
  "status": "completed",
  "started_at": 1703355405,
  "expires_at": null,
  "cancelled_at": null,
  "failed_at": null,
  "completed_at": 1703355420,
  "last_error": null,
  "model": "gpt-4",
  "instructions": "Please address the user as Jane Doe. The user has a premium account.",
  "tools": [
    {
      "type": "code_interpreter"
    }
  ],
  "file_ids": [],
  "metadata": {}
}

This PR makes RunObject.expires_at nullable.

cc @schnerd