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_dag_runs()
        response = self.client.post(
            "api/v1/dags/~/dagRuns/list", json=payload, environ_overrides={'REMOTE_USER': "test"}
        )
        assert response.status_code == 200
        assert response.json["total_entries"] == len(expected_dag_run_ids)
        dag_run_ids = [dag_run["dag_run_id"] for dag_run in response.json["dag_runs"]]
        assert dag_run_ids == expected_dag_run_ids
    def _create_dag_runs(self):
        dates = [
            '2020-06-10T18:00:00+00:00',
            '2020-06-11T18:00:00+00:00',
            '2020-06-12T18:00:00+00:00',
            '2020-06-13T18:00:00+00:00',
            '2020-06-14T18:00:00+00:00',
            '2020-06-15T18:00:00Z',
            '2020-06-16T18:00:00Z',
            '2020-06-17T18:00:00Z',
            '2020-06-18T18:00:00Z',
            '2020-06-19T18:00:00Z',
        ]
        dag = DagModel(dag_id="TEST_DAG_ID")
        dag_runs = [
            DagRun(
                dag_id="TEST_DAG_ID",
                run_id="TEST_START_EXEC_DAY_1" + str(i),
                run_type=DagRunType.MANUAL,
                execution_date=timezone.parse(dates[i]),
                start_date=timezone.parse(dates[i]),
                external_trigger=True,