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
"TEST_DAG_RUN_ID8",
                    "TEST_DAG_RUN_ID9",
                    "TEST_DAG_RUN_ID10",
                ],
            ),
            ("api/v1/dags/TEST_DAG_ID/dagRuns?limit=1&offset=5", ["TEST_DAG_RUN_ID6"]),
            ("api/v1/dags/TEST_DAG_ID/dagRuns?limit=1&offset=1", ["TEST_DAG_RUN_ID2"]),
            (
                "api/v1/dags/TEST_DAG_ID/dagRuns?limit=2&offset=2",
                ["TEST_DAG_RUN_ID3", "TEST_DAG_RUN_ID4"],
            ),
        ]
    )
    def test_handle_limit_and_offset(self, url, expected_dag_run_ids):
        self._create_dag_runs(10)
        response = self.client.get(url, environ_overrides={'REMOTE_USER': "test"})
        assert response.status_code == 200
        assert response.json["total_entries"] == 10
        dag_run_ids = [dag_run["dag_run_id"] for dag_run in response.json["dag_runs"]]
        assert dag_run_ids == expected_dag_run_ids
    def test_should_respect_page_size_limit(self):
        self._create_dag_runs(200)
        response = self.client.get(
            "api/v1/dags/TEST_DAG_ID/dagRuns", environ_overrides={'REMOTE_USER': "test"}
        )
        assert response.status_code == 200
        assert response.json["total_entries"] == 200
        assert len(response.json["dag_runs"]) == 100  
    @conf_vars({("api", "maximum_page_limit"): "150"})
    def test_should_return_conf_max_if_req_max_above_conf(self):