0-output commitment tx with 0-reserve

[TheBlueMatt]

If we accept 0 reserve from a counterparty, it may be possible to have all HTLCs rounded to dust, plus the balances from both sides rounded to dust. Such a commitment tx would be un-broadcastable, as its consensus-invalid, though that commitment tx is also useless as the full channel balance will be burned to fee, there's not really a reason either party would care about just going back to the previous state, as long as the initial state had some output.

We can also only hit it on one side because we only accept, never send, 0 reserves. See-also lightning/bolts#959