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_task_instance_endpoint.py
delete_user(cls.app, username="test")  
        cls.app = app.create_app(testing=True)  
    def setUp(self) -> None:
        self.default_time = DEFAULT_DATETIME_1
        self.ti_init = {
            "execution_date": self.default_time,
            "state": State.RUNNING,
        }
        self.ti_extras = {
            "start_date": self.default_time + dt.timedelta(days=1),
            "end_date": self.default_time + dt.timedelta(days=2),
            "pid": 100,
            "duration": 10000,
            "pool": "default_pool",
            "queue": "default_queue",
            "job_id": 0,
        }
        self.client = self.app.test_client()  
        clear_db_runs()
        clear_db_sla_miss()
        DagBag(include_examples=True, read_dags_from_db=False).sync_to_db()
        self.dagbag = DagBag(include_examples=True, read_dags_from_db=True)
    def create_task_instances(