Triggering a dagrun with naive datetime raises an HTML page instead of JSON

**Apache Airflow version**:2.0


**What happened**:

An HTML page was returned instead of json when the REST API was triggered with a naive datetime

**What you expected to happen**:

I expected it to raise error status 400 indicating that it's a bad request

**How to reproduce it**:
1. Start the web server and scheduler inside breeze.
2. Make a post request to this endpoint: http://localhost:28080/api/v1/dags/example_bash_operator/dagRuns
 to trigger dagruns using this request body: {"execution_date": "2020-11-09T16:25:56.939143"}. or any naive datetime
3. See that an HTML is raised instead of Json

tests/api_connexion/endpoints/test_dag_run_endpoint.py
self._create_test_dag_run()
        response = self.client.post(
            "api/v1/dags/TEST_DAG_ID/dagRuns",
            json={
                "dag_run_id": "TEST_DAG_RUN_ID_1",
                "execution_date": self.default_time,
            },
            environ_overrides={'REMOTE_USER': "test"},
        )
        self.assertEqual(response.status_code, 409, response.data)
        self.assertEqual(
            response.json,
            {
                "detail": "DAGRun with DAG ID: 'TEST_DAG_ID' and "
                "DAGRun ID: 'TEST_DAG_RUN_ID_1' already exists",
                "status": 409,
                "title": "Conflict",
                "type": EXCEPTIONS_LINK_MAP[409],
            },
        )
    def test_should_raises_401_unauthenticated(self):
        response = self.client.post(
            "api/v1/dags/TEST_DAG_ID/dagRuns",
            json={
                "dag_run_id": "TEST_DAG_RUN_ID_1",
                "execution_date": self.default_time,
            },
        )
        assert_401(response)