check Buffer validity

Summary:
if a plasma theme doesn't provide shadow graphics,
the buffer will be a nullptr as would be created
from an invalid image and would crash

BUG:381953

Test Plan: no crash anymore when using a theme that doesn't provide shadows

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel, Frameworks

Tags: Plasma on Wayland, Frameworks

Differential Revision: https://phabricator.kde.org/D6523