The regression was introduced in d8857f21ac. The original change was
meant to fix support for SVG icons, but failed to take into account
a valid QIcon with no sizes, but which is also unable to create
a pixmap for the requested size.
Task-number: QTBUG-58344
Change-Id: I7ac1dbfaf6e3dab8581fe4b33c814e2517fcdba8
Reviewed-by: Andy Shaw <andy.shaw@qt.io>