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
self, _, task_instances, update_extras, single_dag_run, payload, expected_ti_count, session
    ):
        self.create_task_instances(
            session,
            update_extras=update_extras,
            task_instances=task_instances,
            single_dag_run=single_dag_run,
        )
        response = self.client.post(
            "/api/v1/dags/~/dagRuns/~/taskInstances/list",
            environ_overrides={"REMOTE_USER": "test"},
            json=payload,
        )
        self.assertEqual(response.status_code, 200, response.json)
        self.assertEqual(expected_ti_count, response.json["total_entries"])
        self.assertEqual(expected_ti_count, len(response.json["task_instances"]))
    @parameterized.expand(
        [
            (
                "with dag filter",
                {"dag_ids": ["example_python_operator", "example_skip_dag"]},
                15,
                15,
            ),
        ],
    )
    @provide_session
    def test_should_respond_200_dag_ids_filter(self, _, payload, expected_ti, total_ti, session):