Assertion prevents generation of messages with empty payload

[rssen]

The assertion in line 383 of typevalue.py (assert self._value) prevents the generation of a message with empty payload as it raises an AssertionError when encountering an empty bytes-object.
I'd suggest changing it to assert self._value is not None.

According to our UDP spec, it is possible to create a valid UDP Datagram with empty payload by setting the Length field to 8.