Fix as_slice and as_mut_slice for empty Buffers and ArrayBuffers

[jrose-signal]

These buffers may have NULL as their address, which slice::from_raw_parts does not allow.

[kjvalencik]

@jrose-signal In what situations will Node have a null pointer for the backing buffer? I rolled back the changes to src/types/binary.rs and the tests still passed on v14.15.4.

Is this something that is "working" in some cases but, unsound and potentially not working in others (e.g., if an enum is relying on a niche).

[kjvalencik]

Hmm, I guess it doesn't matter if we can reproduce or not. In either case it's clearly relying on UB. From the N-API docs:

If length is 0, this may be NULL or any other pointer value.

[jrose-signal]

The newly-added tests do fail for me with Node v12.13.0 (at least, that's what I have as which node). I suspect there's also an optimization aspect to this; immediately to_vec() on an empty buffer's slice worked fine for me but doing more complicated work caused problems.