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_task_instance_endpoint.py
"/taskInstances?queue=test_queue_1,test_queue_2"
                ),
                2,
            ),
            (
                "test queue filter ~",
                [
                    {"queue": "test_queue_1"},
                    {"queue": "test_queue_2"},
                    {"queue": "test_queue_3"},
                ],
                True,
                "/api/v1/dags/~/dagRuns/~/taskInstances?queue=test_queue_1,test_queue_2",
                2,
            ),
        ]
    )
    @provide_session
    def test_should_respond_200(self, _, task_instances, update_extras, url, expected_ti, session):
        self.create_task_instances(
            session,
            update_extras=update_extras,
            task_instances=task_instances,
        )
        response = self.client.get(url, environ_overrides={"REMOTE_USER": "test"})
        self.assertEqual(response.status_code, 200)
        self.assertEqual(response.json["total_entries"], expected_ti)
        self.assertEqual(len(response.json["task_instances"]), expected_ti)
    @provide_session
    def test_should_respond_200_for_dag_id_filter(self, session):