Make input vendor and product fields optional

[FortStatement]

get_inputs calls started failing on my machine because wlr_virtual_keyboard_v1 doesn't have vendor or product fields (cargo test fails as well). swaymsg treats these as optional in practice and uses 0 by default for the values.

[JayceFayne]

Looking at sways ipc code it seems like these fields are only set for libinput devices. Like the libinput field itself these should indeed be optional. Can you pls group them together and add a doc comment for the vendor and product fields?

[JayceFayne]

Thanks!