book/ffi: nullable pointer cleanup
Expand the "nullable pointer optimization" section with a code example. Fixes #34250.
I also noticed that many of the examples use the libc crate just for types such as `c_char` and `c_int`, which are now available through `std::os::raw`. I changed the ones that don't need to rely on libc. I'm glad to revert that part of the commit if it's unwanted churn.