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
"api/v1/dags/TEST_DAG_ID/dagRuns?execution_date_lte=2020-06-13T18:00:00+00:00",
                [
                    "TEST_START_EXEC_DAY_10",
                    "TEST_START_EXEC_DAY_11",
                    "TEST_START_EXEC_DAY_12",
                    "TEST_START_EXEC_DAY_13",
                ],
            ),
            (
                "api/v1/dags/TEST_DAG_ID/dagRuns?execution_date_gte=2020-06-16T18:00:00+00:00",
                [
                    "TEST_START_EXEC_DAY_16",
                    "TEST_START_EXEC_DAY_17",
                    "TEST_START_EXEC_DAY_18",
                    "TEST_START_EXEC_DAY_19",
                ],
            ),
        ]
    )
    @provide_session
    def test_date_filters_gte_and_lte(self, url, expected_dag_run_ids, session):
        dagrun_models = self._create_dag_runs()
        session.add_all(dagrun_models)
        session.commit()
        response = self.client.get(url, 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):