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.dag2_id}/details", environ_overrides={'REMOTE_USER': "test"}
        )
        assert response.status_code == 200
        expected = {
            "catchup": True,
            "concurrency": 16,
            "dag_id": "test_dag2",
            "dag_run_timeout": None,
            "default_view": "tree",
            "description": None,
            "doc_md": None,
            "fileloc": __file__,
            "is_paused": None,
            "is_subdag": False,
            "orientation": "LR",
            "owners": [],
            "schedule_interval": {
                "__type": "TimeDelta",
                "days": 1,
                "microseconds": 0,
                "seconds": 0,
            },
            "start_date": "2020-06-15T00:00:00+00:00",
            "tags": None,
            "timezone": "Timezone('UTC')",
        }
        assert response.json == expected
    def test_should_respond_200_serialized(self):