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
del name
        dag_instance = DagModel(dag_id="TEST_DAG_ID")
        session.add(dag_instance)
        session.commit()
        response = self.client.post(
            "api/v1/dags/TEST_DAG_ID/dagRuns", json=request_json, environ_overrides={'REMOTE_USER': "test"}
        )
        self.assertEqual(response.status_code, 200)
        self.assertEqual(
            {
                "conf": {},
                "dag_id": "TEST_DAG_ID",
                "dag_run_id": response.json["dag_run_id"],
                "end_date": None,
                "execution_date": response.json["execution_date"],
                "external_trigger": True,
                "start_date": response.json["start_date"],
                "state": "running",
            },
            response.json,
        )
    def test_response_404(self):