ffi/unsafe: fix `make-sized-byte-string` on a #f argument

In particular, a #f argument can make sense if the length is 0.
Technically, a byte string's byte array is supposed to be nul-terminated,
but many uses of byte strings get away without that terminator. I've
adjust the documentation to note that `bytes-copy` will work with a
non-terminated byte string.

Merge to v6.1.1