Fix wlr.Output make, model, serial fields

In the `wlr/types/wlr_output.h`, `wlr_output`'s `make`, `model`, and `serial` fields are all defined as `char *` that may be `NULL`, but currently in these bindings, they're defined as a nullable pointer to a C string (`?*[*:0]u8`), which is incorrect. This MR makes these fields into nullable C strings (`?[*:0]u8`).