Only display old and new versions when both are available

Summary:
Sometimes, availableVersion isn't set when the i18n() string is built, leading to ugly
"argument not found" errors. This is an attempt to fix it by not even displaying the
string until both are set.

Test Plan: Hard to reproduce, but in my testing, it works.

Reviewers: apol, Discover Software Store

Reviewed By: apol, Discover Software Store

Subscribers: pino, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D23250