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
"is_paused": False,
                        "is_subdag": False,
                        "owners": [],
                        "root_dag_id": None,
                        "schedule_interval": {
                            "__type": "CronExpression",
                            "value": "2 2 * * *",
                        },
                        "tags": [],
                    },
                    {
                        "dag_id": "TEST_DAG_2",
                        "description": None,
                        "fileloc": "/tmp/dag_2.py",
                        "is_paused": False,
                        "is_subdag": False,
                        "owners": [],
                        "root_dag_id": None,
                        "schedule_interval": {
                            "__type": "CronExpression",
                            "value": "2 2 * * *",
                        },
                        "tags": [],
                    },
                ],
                "total_entries": 2,
            },
            response.json,
        )
    def test_should_respond_200_with_granular_dag_access(self):