Here are some bugs in dag details endpoint.

1. Calling this endpoint `dags/{dag_id}/details` returns an error due to `doc_md` field that's not nullable.

2. The dag details endpoint `GET /dags/{dag_id}/details` does not return a json if dag does not exist.

3. This endpoint does not return a `file_token` which is needed to get the source code of a DAG. And also expected judging from the documentation.

**Apache Airflow version**: 2.0


**What happened**:
1. Calling this endpoint GET /dags/{dag_id}/details returns an error
2. When the above is fixed and you look for the details of a dag that does not exist, this endpoint returns a webpage instead of json.
3. Calling the endpoint with a dag ID that exists doesn't return a file_token

**What you expected to happen**:
1. I expected it to return the details of the dag
2. I expected it to return a json indicating that the dag was not found if dag doesn't exist
3. I expected it to return a file_token in the list of fields when the dag exists

**How to reproduce it**:
Run airflow in breeze.
Make a request to `/dags/{dag_id}/details`. It will return an error.
Go to openapi.yaml and set `doc_md` to be nullable.
Make another request to see that it returns a result without a file_token.
Change the dag id to a dag ID that does not exist.
Make another request and see that it returns a webpage.

tests/api_connexion/endpoints/test_dag_endpoint.py
patch_body = {
            "is_paused": True,
            "schedule_interval": {
                "__type": "CronExpression",
                "value": "1 1 * * *",
            },
        }
        dag_model = self._create_dag_model()
        response = self.client.patch(f"/api/v1/dags/{dag_model.dag_id}", json=patch_body)
        self.assertEqual(response.status_code, 400)
        self.assertEqual(
            response.json,
            {
                'detail': "Property is read-only - 'schedule_interval'",
                'status': 400,
                'title': 'Bad Request',
                'type': EXCEPTIONS_LINK_MAP[400],
            },
        )
    def test_should_respond_404(self):
        response = self.client.get("/api/v1/dags/INVALID_DAG", environ_overrides={'REMOTE_USER': "test"})
        self.assertEqual(response.status_code, 404)
    @provide_session
    def _create_dag_model(self, session=None):