Prevent empty decommits

[ch1bo]

Follow-up of #1057

Why

After making incremental decommits possible in #1057, we realized that the original specification is not preventing "empty decommit" requests. That means, that anyone can request to decommit nothing. While this is no safety problem, it can lead to annoying situations where one participant spams the head with empty decrements (on the L1).

What

Instead of detecting the spam, we want to prevent this situation altogether by rejecting ReqDec requests whose transactions don't have any outputs.

How

This is a BehaviorSpec test that covers this:

      it "prevents empty decommits" $
        shouldRunInSim $ do
          withSimulatedChainAndNetwork $ \chain ->
            withHydraNode aliceSk [bob] chain $ \n1 ->
              withHydraNode bobSk [alice] chain $ \n2 -> do
                openHead chain n1 n2

                send n1 (Decommit $ SimpleTx 1 mempty mempty)
                waitUntilMatch [n1] $ \case
                  DecommitInvalid{headId} | headId == testHeadId -> True
                  _ -> False

We might want to ensure though that also requests of other nodes are rejected.