pw: data: Check `data` field for NULL and return as an `Option`.
Fixes potential UB by now checking a Data
s data
pointer for NULL, and returning the created slice in an Option
, as the field is only valid sometimes.
Related to #32.
While I was inside the file, I also added bindings for a couple struct fields, did some cleanup and moved it to the libspa crate, as there are only spa types involved.