kcm_energyinfo: Allow changing of the timespan if no data available

Summary:
Without this, it's only possible to change the timespan if data is available.
Which means that if there is not data available for i.e. the last 24 hours,
you can't switch back to a longer span.

Test Plan: I can change to a timespan with data.

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11712