Only display the DVC add prompt if there is anything to add

[guysmoilov]

Currently, it will display always display
========== Make your selection, Press "h" for help ==========
even if there is no selection to make since the list of files is empty

https://github.com/DAGsHub/fds/blob/a8fea54f59131d3ddea4df5184adeee3ecc9998f/fds/services/dvc_service.py#L119

[guysmoilov]

Fixed in #46