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_dag_run_endpoint.py
'external_trigger': True,
                    'start_date': self.default_time,
                    'conf': {},
                },
            ],
            "total_entries": 2,
        }
    @parameterized.expand(
        [
            (
                {"dag_ids": ["TEST_DAG_ID"], "page_offset": -1},
                "-1 is less than the minimum of 0 - 'page_offset'",
            ),
            ({"dag_ids": ["TEST_DAG_ID"], "page_limit": 0}, "0 is less than the minimum of 1 - 'page_limit'"),
            ({"dag_ids": "TEST_DAG_ID"}, "'TEST_DAG_ID' is not of type 'array' - 'dag_ids'"),
            ({"start_date_gte": "2020-06-12T18"}, "{'start_date_gte': ['Not a valid datetime.']}"),
        ]
    )
    def test_payload_validation(self, payload, error):
        self._create_test_dag_run()
        response = self.client.post(
            "api/v1/dags/~/dagRuns/list", json=payload, environ_overrides={'REMOTE_USER': "test"}
        )
        assert response.status_code == 400
        assert error == response.json.get("detail")
    def test_should_raises_401_unauthenticated(self):
        self._create_test_dag_run()
        response = self.client.post("api/v1/dags/~/dagRuns/list", json={"dag_ids": ["TEST_DAG_ID"]})
        assert_401(response)
class TestGetDagRunBatchPagination(TestDagRunEndpoint):