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
response = self.client.get(
            "api/v1/dags/invalid-id/dagRuns/invalid-id", environ_overrides={'REMOTE_USER': "test"}
        )
        assert response.status_code == 404
        expected_resp = {
            'detail': "DAGRun with DAG ID: 'invalid-id' and DagRun ID: 'invalid-id' not found",
            'status': 404,
            'title': 'DAGRun not found',
            'type': EXCEPTIONS_LINK_MAP[404],
        }
        assert expected_resp == response.json
    @provide_session
    def test_should_raises_401_unauthenticated(self, session):
        dagrun_model = DagRun(
            dag_id="TEST_DAG_ID",
            run_id="TEST_DAG_RUN_ID",
            run_type=DagRunType.MANUAL,
            execution_date=timezone.parse(self.default_time),
            start_date=timezone.parse(self.default_time),
            external_trigger=True,
        )
        session.add(dagrun_model)
        session.commit()
        response = self.client.get("api/v1/dags/TEST_DAG_ID/dagRuns/TEST_DAG_RUN_ID")
        assert_401(response)
class TestGetDagRuns(TestDagRunEndpoint):