wayland: fix missing nullable markers on wl_data_device's event args
wl_data_device.selection's id argument is nullable in the real protocol
(sent NULL whenever there's no current clipboard offer, e.g. right after
gaining keyboard focus with nothing yet copied) — transcribed here as a
plain non-nullable "o", which crashes libwayland's own argument
validation the moment a real compositor sends that NULL ("NULL object
received on non-nullable type, message selection(o)").
Fixes the same gap on every other nullable argument in this file
(wl_data_source.target's mime_type, wl_data_device.enter/start_drag/
set_selection's object args, wl_data_offer.accept's mime_type) — none of
the others have surfaced yet since they're all either unused padding or
requests we only ever call with non-null arguments ourselves, but the
signature string is what wl_proxy_marshal_flags itself uses to interpret
outgoing varargs, not just something the receiver validates, so they were
still latent bugs.