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
response = self.client.get(f"/api/v1/dags/{self.dag_id}/details")
        assert_401(response)
    def test_should_raise_404_when_dag_is_not_found(self):
        response = self.client.get(
            "/api/v1/dags/non_existing_dag_id/details", environ_overrides={'REMOTE_USER': "test"}
        )
        assert response.status_code == 404
        self.assertEqual(
            response.json,
            {
                'detail': 'The DAG with dag_id: non_existing_dag_id was not found',
                'status': 404,
                'title': 'DAG not found',
                'type': EXCEPTIONS_LINK_MAP[404],
            },
        )
class TestGetDags(TestDagEndpoint):
    def test_should_respond_200(self):
        self._create_dag_models(2)
        response = self.client.get("api/v1/dags", environ_overrides={'REMOTE_USER': "test"})
        assert response.status_code == 200
        self.assertEqual(
            {
                "dags": [
                    {
                        "dag_id": "TEST_DAG_1",
                        "description": None,
                        "fileloc": "/tmp/dag_1.py",