test ant/term ↗

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.