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
delete_user(cls.app, username="test")  
        delete_user(cls.app, username="test_granular_permissions")  
        delete_user(cls.app, username="test_no_permissions")  
    def setUp(self) -> None:
        self.client = self.app.test_client()  
        self.default_time = "2020-06-11T18:00:00+00:00"
        self.default_time_2 = "2020-06-12T18:00:00+00:00"
        clear_db_runs()
        clear_db_dags()
    def tearDown(self) -> None:
        clear_db_runs()
        
    def _create_test_dag_run(self, state='running', extra_dag=False, commit=True):
        dag_runs = []
        dags = [DagModel(dag_id="TEST_DAG_ID")]
        dagrun_model_1 = DagRun(
            dag_id="TEST_DAG_ID",
            run_id="TEST_DAG_RUN_ID_1",
            run_type=DagRunType.MANUAL,
            execution_date=timezone.parse(self.default_time),
            start_date=timezone.parse(self.default_time),
            external_trigger=True,
            state=state,