Status bar: Hide the space free bar when size is unknown

Summary:
BUG: 420027
BUG: 422226
FIXED-IN: 20.08

Test Plan:

Reviewers: Dolphin, ngraham, broulik, VDG, elvisangelaccio

Reviewed By: Dolphin, ngraham, VDG, elvisangelaccio

Subscribers: elvisangelaccio, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D29213