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
state='success',
            )
            for i in range(len(dates))
        ]
        with create_session() as session:
            session.add_all(dag_runs)
            session.add(dag)
        return dag_runs
    @parameterized.expand(
        [
            (
                {"end_date_gte": f"{(timezone.utcnow() + timedelta(days=1)).isoformat()}"},
                [],
            ),
            (
                {"end_date_lte": f"{(timezone.utcnow() + timedelta(days=1)).isoformat()}"},
                ["TEST_DAG_RUN_ID_1"],
            ),
        ]
    )
    def test_end_date_gte_lte(self, payload, expected_dag_run_ids):
        self._create_test_dag_run('success')  
        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"] if dag_run]
        assert dag_run_ids == expected_dag_run_ids
class TestPostDagRun(TestDagRunEndpoint):