shell: Don't assign keyboard focus for seats without keyboard
shell: Don't assign keyboard focus for seats without keyboard

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=73570

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