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
@provide_session
    def test_should_respond_204(self, session):
        session.add_all(self._create_test_dag_run())
        session.commit()
        response = self.client.delete(
            "api/v1/dags/TEST_DAG_ID/dagRuns/TEST_DAG_RUN_ID_1", environ_overrides={'REMOTE_USER': "test"}
        )
        self.assertEqual(response.status_code, 204)
        
        response = self.client.get(
            "api/v1/dags/TEST_DAG_ID/dagRuns/TEST_DAG_RUN_ID_1", environ_overrides={'REMOTE_USER': "test"}
        )
        self.assertEqual(response.status_code, 404)
    def test_should_respond_404(self):