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