[Touchpad KCM] Fix kded crashing at startup

Summary:
When XcbAtom::atom() is called (by kded on key press other than Kded Keyboard shortcuts), m_connection seems to be having null or 0x0 value.
Small check on m_connection solves the issue for me.

BUG: 407614

Test Plan: kded5 do not crashed at startup.

Reviewers: romangg, davidedmundson

Reviewed By: romangg

Subscribers: fvogt, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21278