[rtcweb] Sending of zero-length messages over data channels

[Justin Uberti <[e-mail]>]

In the existing data channel protocol, it's not possible to send
zero-length messages, because SCTP prohibits it.

While this may seem to be an academic point, it does represent a divergence
from WebSockets, and this difference could be meaningful to applications
who use empty messages for keepalives. Or maybe the Yo app.

Since we can't send a zero-length SCTP message, I propose we add a
PPID_EMPTY value or something similar to indicate a message with empty
content.