[kcm] Handle empty shortcut in property fetch

Summary:
.first() wasn't guarded with a check of
the list size.

BUG: 416397
FIXED-IN: 5.18.0

Reviewers: #plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26772