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
'state': 'running',
                    'execution_date': self.default_time_2,
                    'external_trigger': True,
                    'start_date': self.default_time,
                    'conf': {},
                },
            ],
            "total_entries": 2,
        }
    def test_should_return_all_with_tilde_as_dag_id_and_all_dag_permissions(self):
        self._create_test_dag_run(extra_dag=True)
        expected_dag_run_ids = ['TEST_DAG_ID', 'TEST_DAG_ID', "TEST_DAG_ID_3", "TEST_DAG_ID_4"]
        response = self.client.get("api/v1/dags/~/dagRuns", environ_overrides={'REMOTE_USER': "test"})
        assert response.status_code == 200
        dag_run_ids = [dag_run["dag_id"] for dag_run in response.json["dag_runs"]]
        assert dag_run_ids == expected_dag_run_ids
    def test_should_return_accessible_with_tilde_as_dag_id_and_dag_level_permissions(self):
        self._create_test_dag_run(extra_dag=True)
        expected_dag_run_ids = ['TEST_DAG_ID', 'TEST_DAG_ID']
        response = self.client.get(
            "api/v1/dags/~/dagRuns", environ_overrides={'REMOTE_USER': "test_granular_permissions"}
        )
        assert response.status_code == 200
        dag_run_ids = [dag_run["dag_id"] for dag_run in response.json["dag_runs"]]
        assert dag_run_ids == expected_dag_run_ids
    def test_should_raises_401_unauthenticated(self):
        self._create_test_dag_run()
        response = self.client.get("api/v1/dags/TEST_DAG_ID/dagRuns")
        assert_401(response)
class TestGetDagRunsPagination(TestDagRunEndpoint):