[libc++][NFC] Refactor __request_unbuffered_mode (#168950)
It's a bit awkward to pass the char* and the size to the function, which
then only decides whether it should be a no-op with it. Instead, only
call the function when the pointer is null and n is 0, which matches
more closely what the standard says with respect to calling pubsetbuf(0, 0).
