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
environ_overrides={"REMOTE_USER": "test"},
        )
        self.assertEqual(response.status_code, 200)
        self.assertDictEqual(
            response.json,
            {
                "dag_id": "example_python_operator",
                "duration": 10000.0,
                "end_date": "2020-01-03T00:00:00+00:00",
                "execution_date": "2020-01-01T00:00:00+00:00",
                "executor_config": "{}",
                "hostname": "",
                "max_tries": 0,
                "operator": "PythonOperator",
                "pid": 100,
                "pool": "default_pool",
                "pool_slots": 1,
                "priority_weight": 6,
                "queue": "default_queue",
                "queued_when": None,
                "sla_miss": None,
                "start_date": "2020-01-02T00:00:00+00:00",
                "state": "running",
                "task_id": "print_the_context",
                "try_number": 0,
                "unixname": getpass.getuser(),
            },
        )
    @provide_session
    def test_should_respond_200_task_instance_with_sla(self, session):