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
@classmethod
    def setUpClass(cls) -> None:
        super().setUpClass()
        with conf_vars({("api", "auth_backend"): "tests.test_utils.remote_user_api_auth_backend"}):
            cls.app = app.create_app(testing=True)  
        create_user(
            cls.app,  
            username="test",
            role_name="Test",
            permissions=[
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_RUN),
                (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
                (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_TASK_INSTANCE),
            ],
        )
        create_user(cls.app, username="test_no_permissions", role_name="TestNoPermissions")  
    @classmethod
    def tearDownClass(cls) -> None: