Filter Empty Updates from YDoc.observe_after_transaction

[Waidhoferj]

Fixes #98

observe_after_transaction callbacks should only trigger if there are contentful updates to the CRDT. There are certain operations that need a YTransaction to complete, but don't mutate any underlying state. This caused empty updates in observe_after_transaction, which is a meaningless event.

To avoid propagating these noops, @davidbrochart and I added this boilerplate to our callbacks:

def send_updates(self, txn_event: Y.AfterTransactionEvent):
        update = txn_event.get_update()
        if update != b'\x00\x00':
            send_update(update) 

Since we expect this behavior across the board, this PR filters out empty updates before the callback is triggered, so we will no longer have to add this check on a per-case basis.