[Touchpad KCM] Do not crash in case there is no touchpad

Summary:
BUG: 408325
FIXED-IN: 5.16.2

Reviewers: ngraham, romangg, atulbi, #plasma, davidedmundson

Reviewed By: ngraham, romangg, #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21884