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
{
                'dag_id': 'example_python_operator',
                'dag_run_id': 'TEST_DAG_RUN_ID_1',
                'execution_date': '2020-01-02T00:00:00+00:00',
                'task_id': 'sleep_for_0',
            },
            {
                'dag_id': 'example_python_operator',
                'dag_run_id': 'TEST_DAG_RUN_ID_2',
                'execution_date': '2020-01-03T00:00:00+00:00',
                'task_id': 'sleep_for_1',
            },
            {
                'dag_id': 'example_python_operator',
                'dag_run_id': 'TEST_DAG_RUN_ID_3',
                'execution_date': '2020-01-04T00:00:00+00:00',
                'task_id': 'sleep_for_2',
            },
            {
                'dag_id': 'example_python_operator',
                'dag_run_id': 'TEST_DAG_RUN_ID_4',
                'execution_date': '2020-01-05T00:00:00+00:00',
                'task_id': 'sleep_for_3',
            },
        ]
        for task_instance in expected_response:
            self.assertIn(task_instance, response.json["task_instances"])
        self.assertEqual(5, len(response.json["task_instances"]))
        self.assertEqual(0, failed_dag_runs, 0)
    def test_should_raises_401_unauthenticated(self):