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
@provide_session
    @mock.patch('airflow.api_connexion.endpoints.task_instance_endpoint.set_state')
    def test_should_assert_call_mocked_api(self, mock_set_state, session):
        self.create_task_instances(session)
        mock_set_state.return_value = (
            session.query(TaskInstance).filter(TaskInstance.task_id == "print_the_context").all()
        )
        response = self.client.post(
            "/api/v1/dags/example_python_operator/updateTaskInstancesState",
            environ_overrides={'REMOTE_USER': "test"},
            json={
                "dry_run": True,
                "task_id": "print_the_context",
                "execution_date": DEFAULT_DATETIME_1,
                "include_upstream": True,
                "include_downstream": True,
                "include_future": True,
                "include_past": True,
                "new_state": "failed",
            },
        )
        assert response.status_code == 200
        assert response.json == {
            'task_instances': [
                {
                    'dag_id': 'example_python_operator',
                    'dag_run_id': 'TEST_DAG_RUN_ID',
                    'execution_date': '2020-01-01T00:00:00+00:00',
                    'task_id': 'print_the_context',
                }