Empty updates

[davidbrochart]

@Waidhoferj There is this code in your drawing example:

ypy/examples/drawing/client.py

Lines 28 to 31 in 792dc96

	# Sometimes transactions don't write, which means updates are empty.
	# We only care about updates with meaningful mutations.
	if update != b'\x00\x00':
	self.send_q.put_nowait(update)

Why are empty updates emitted? This creates issues here and there.