Show previews on encrypted filesystems

Summary:
Instead of skipping generating previews on encrypted filesystems, do create them but don't cache.

BUG: 411919
FIXED-IN: 5.70

Reviewers: ivan, Frameworks, dfaure, ngraham

Reviewed By: ivan, ngraham

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28746