shell: run surface bindings only when focus surface is not NULL
shell: run surface bindings only when focus surface is not NULL

This fixes the crash when move, rotate or resize binding is activated
while exposay effect is active.

Steps to reproduce:
- activate exposay
- try to rotate the surface with mod + right mouse button
- crash

Closes: https://bugs.freedesktop.org/show_bug.cgi?id=72885

sir-murray/weston,Tarnyko/weston-xdg_surface_present,giucam/weston,Gnurou/weston,Gnurou/weston,kwm81/weston,kwm81/weston,xorgy/weston,krezovic/weston,udoprog/weston,eyolfson/weston,Fantu/compositor-spice,jonnylamb/weston,mchalupa/weston,Fantu/compositor-spice,krezovic/weston,sir-murray/weston,eyolfson/weston,Fantu/compositor-spice,udoprog/weston,xorgy/weston,Tarnyko/weston-xdg_surface_present,jonnylamb/weston,mchalupa/weston,giucam/weston