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
},
            {
                "execution_date": DEFAULT_DATETIME_1 + dt.timedelta(days=4),
                "state": State.RUNNING,
            },
        ]
        self.create_task_instances(
            session,
            dag_id=dag_id,
            single_dag_run=False,
            task_instances=task_instances,
            update_extras=False,
            dag_run_state=State.FAILED,
        )
        response = self.client.post(
            f"/api/v1/dags/{dag_id}/clearTaskInstances",
            environ_overrides={"REMOTE_USER": "test"},
            json=payload,
        )
        failed_dag_runs = (
            session.query(DagRun).filter(DagRun.state == "failed").count()  
        )
        self.assertEqual(200, response.status_code)
        expected_response = [
            {
                'dag_id': 'example_python_operator',
                'dag_run_id': 'TEST_DAG_RUN_ID_0',
                'execution_date': '2020-01-01T00:00:00+00:00',
                'task_id': 'print_the_context',
            },