Reject fee udpate post-shutdown when no HTLCs remain

[TheBlueMatt]

This could throw off our closing signed stuff, and its just confusing, and already forbidden, so we should just forbid it too. Spec says:

if no HTLCs remain in either commitment transaction:
    MUST NOT send any update message after a shutdown.

[dunxen]

So if this is something manageable, I can grab it and go with it as I'd like to dig into these bits.

[TheBlueMatt]

We may actually just change the spec here - its not clear that this accomplishes anything in the spec aside from complication - https://github.com/lightning/bolts/pull/972