wayland: Fix missing relative motion events

Use isNull on QSizeF to check for a zero delta instead of comparing it with a default-constructed QSizeF, which in practice initializes to (-1.0,-1.0). This caused relative motion events to be omitted if the delta happened to be equal to (-1.0,-1.0), causing mouse jumping in some applications.

I couldn't find any reported bugs regarding this. Here is a video of the problem that this fixes: http://www.fastquake.com/files/kwin_wayland-teardown-mouse.mp4