Add an isNull() check before setting wheter QIcon is a mask

Summary: To avoid stepping on a null QIcon. This can happen when calling ::fromTheme with a icon name that doesn't exist on the platform

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D10482