WorkbookItem.update() always sends an empty <dataAccelerationConfig/> even when never set

[jacalata]

WorkbookItem.__init__ defaults data_acceleration_config to a dict with
all-None values:

self.data_acceleration_config = {
    "acceleration_enabled": None,
    "accelerate_now": None,
    "last_updated_at": None,
    "acceleration_status": None,
}

request_factory.py's update_req checks if workbook_item.data_acceleration_config:
to decide whether to emit <dataAccelerationConfig/>. Since the dict itself
is always truthy (non-empty), this is always True — every workbooks.update()
call emits an empty <dataAccelerationConfig/> tag, even when the caller
never touched acceleration settings.