Refactor spot api list_orders method

[lorenzourera]

There is a need to refactor the list_orders method under the spot api.

Based on the API documentation, we should be allowed to make a request wherein currency_pair is None or not specified as long as the status=finished as seen in the screenshot below

Solution here is to replace lines 1794 - 1799

With this

if self.api_client.client_side_validation and (
            'currency_pair' not in local_var_params or local_var_params['currency_pair'] is None
        ) and local_var_params['status'] == 'open':
            raise ApiValueError(
                "Missing the required parameter `currency_pair` when calling `list_orders` with status 'open'"
            )