protocol: Add missing allow-null to data_device.enter data_offer field
protocol: Add missing allow-null to data_device.enter data_offer field

Fixes bug 51889.

bitfactor/wayland,tulcod/wayland,bitfactor/wayland,eyolfson/wayland,abooij/wayland,tulcod/wayland,mchalupa/wayland,bitfactor/wayland,mchalupa/wayland,abooij/wayland,tulcod/wayland,mchalupa/wayland,abooij/wayland,sir-murray/wayland,sir-murray/wayland,eyolfson/wayland,eyolfson/wayland,sir-murray/wayland