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
response = self.client.post(
            "/api/v1/dags/example_python_operator/updateTaskInstancesState",
            environ_overrides={'REMOTE_USER': "test_no_permissions"},
            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 == 403
    def test_should_raise_404_not_found_dag(self):