stream: Add compliant handling of non-blocking read()/write().
In case of empty non-blocking read()/write(), both return None. read() cannot return 0, as that means EOF, so returns another value, and then write() just follows. This is still pretty unexpected, and typical "if not len:" check would treat this as EOF. Well, non-blocking files require special handling! This also kind of makes it depending on POSIX, but well, anything else should emulate POSIX anyway ;-).