proof: fix universe courier termination condition for grouped assets

In this commit, we update the termination condition to account for the
fact that grouped assets have a distinct witness from non-grouped
assets.

Without this, we'll fail to fetch the proof files for grouped assets.

proof/courier.go

@@ -1071,7 +1071,8 @@ func (c *UniverseRpcCourier) ReceiveProof(ctx context.Context,
 
 		// Break if we've reached the genesis point (the asset is the
 		// genesis asset).
-		if transitionProof.Asset.HasGenesisWitness() {
+		if transitionProof.Asset.HasGenesisWitness() ||
+			transitionProof.Asset.HasGenesisWitnessForGroup() {
 			break
 		}