Fix message when no assignments available to an experiment

[dpordomingo]

(context: #56 (comment))

It was strongly requested to configure the API to respond with an empty list when there are no assignments available to an experiment/user, but the frontend is not prepared for that change, so it is needed to fix it;

When the user has no assignments to fill, instead of rising a reasonable error, he obtains a "congrats for filling NaN% experiments, with no possibility to move to any other window.