stream: Don't signal EOF on '' or Buffer(0)

Those values, if passed to the _read() cb, will not signal an EOF.  Only
null or undefined will mark the end of data, and trigger the end event.

However, great care must be taken if you are returning an empty string
or buffer!  There must be some other thing somewhere that will trigger
a read() call, because there will never be a readable event fired later.

This is in preparation for CryptoStreams being ported to streams2, where
it is safe to simply stop reading, because the crypto cycle process will
cause it to read(0) again at some future date.