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
dag_id = "test_dag"
    task_id = "op1"
    dag2_id = "test_dag2"
    @staticmethod
    def clean_db():
        clear_db_runs()
        clear_db_dags()
        clear_db_serialized_dags()
    @classmethod
    def setUpClass(cls) -> None:
        super().setUpClass()
        with conf_vars({("api", "auth_backend"): "tests.test_utils.remote_user_api_auth_backend"}):
            cls.app = app.create_app(testing=True)  
        create_user(
            cls.app,  
            username="test",
            role_name="Test",
            permissions=[
                (permissions.ACTION_CAN_CREATE, permissions.RESOURCE_DAG),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
                (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG),
                (permissions.ACTION_CAN_DELETE, permissions.RESOURCE_DAG),
            ],