Avoid crash if kicker plugin is used outside plasma context

Summary:
Code currently gets the applet interface object passed in explicitly,
from this it gets the configuration via the metaobject system

We always guard the appletConfig not being available, but never the
appletInterface, resulting in crashes if used from qmlscene or any
potential tests.

Test Plan: Used from qmlscene

Reviewers: #plasma, apol, hein

Reviewed By: #plasma, apol, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22818