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
response = self.client.post(
            "api/v1/dags/TEST_DAG_ID/dagRuns",
            json={"dag_run_id": "TEST_DAG_RUN", "execution_date": self.default_time},
            environ_overrides={'REMOTE_USER': "test"},
        )
        self.assertEqual(response.status_code, 404)
        self.assertEqual(
            {
                "detail": "DAG with dag_id: 'TEST_DAG_ID' not found",
                "status": 404,
                "title": "DAG not found",
                "type": EXCEPTIONS_LINK_MAP[404],
            },
            response.json,
        )
    @parameterized.expand(
        [
            (
                "start_date in request json",
                "api/v1/dags/TEST_DAG_ID/dagRuns",
                {
                    "start_date": "2020-06-11T18:00:00+00:00",
                    "execution_date": "2020-06-12T18:00:00+00:00",
                },