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
self._create_dag_runs(200)
        response = self.client.get(
            "api/v1/dags/TEST_DAG_ID/dagRuns?limit=180", environ_overrides={'REMOTE_USER': "test"}
        )
        assert response.status_code == 200
        self.assertEqual(len(response.json["dag_runs"]), 150)
    def _create_dag_runs(self, count):
        dag_runs = [
            DagRun(
                dag_id="TEST_DAG_ID",
                run_id="TEST_DAG_RUN_ID" + str(i),
                run_type=DagRunType.MANUAL,
                execution_date=timezone.parse(self.default_time) + timedelta(minutes=i),
                start_date=timezone.parse(self.default_time),
                external_trigger=True,
            )
            for i in range(1, count + 1)
        ]
        dag = DagModel(dag_id="TEST_DAG_ID")
        with create_session() as session:
            session.add_all(dag_runs)
            session.add(dag)
class TestGetDagRunsPaginationFilters(TestDagRunEndpoint):