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 = self.app.dag_bag.dags['example_python_operator']  
        task = dag.task_dict['print_the_context']
        mock_set_state.assert_called_once_with(
            commit=False,
            downstream=True,
            execution_date=DEFAULT_DATETIME_1,
            future=True,
            past=True,
            state='failed',
            tasks=[task],
            upstream=True,
        )
    def test_should_raises_401_unauthenticated(self):
        response = self.client.post(
            "/api/v1/dags/example_python_operator/updateTaskInstancesState",
            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_401(response)
    def test_should_raise_403_forbidden(self):