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
"queue": "default_queue",
                "queued_when": None,
                "sla_miss": {
                    "dag_id": "example_python_operator",
                    "description": None,
                    "email_sent": False,
                    "execution_date": "2020-01-01T00:00:00+00:00",
                    "notification_sent": False,
                    "task_id": "print_the_context",
                    "timestamp": "2020-01-01T00:00:00+00:00",
                },
                "start_date": "2020-01-02T00:00:00+00:00",
                "state": "running",
                "task_id": "print_the_context",
                "try_number": 0,
                "unixname": getpass.getuser(),
            },
        )
    def test_should_raises_401_unauthenticated(self):
        response = self.client.get(
            "api/v1/dags/example_python_operator/dagRuns/TEST_DAG_RUN_ID/taskInstances/print_the_context",
        )
        assert_401(response)
    def test_should_raise_403_forbidden(self):
        response = self.client.get(
            "api/v1/dags/example_python_operator/dagRuns/TEST_DAG_RUN_ID/taskInstances/print_the_context",
            environ_overrides={'REMOTE_USER': "test_no_permissions"},
        )
        assert response.status_code == 403
class TestGetTaskInstances(TestTaskInstanceEndpoint):