Make Counter app return nil hash initially

example/counter.go

@@ -68,10 +68,14 @@ func (appC *CounterAppContext) AppendTx(tx []byte) ([]types.Event, types.RetCode
 }
 
 func (appC *CounterAppContext) GetHash() ([]byte, types.RetCode) {
-	hash := make([]byte, 32)
-	binary.PutVarint(hash, int64(appC.txCount))
 	appC.hashCount += 1
-	return hash, 0
+	if appC.txCount == 0 {
+		return nil, 0
+	} else {
+		hash := make([]byte, 32)
+		binary.PutVarint(hash, int64(appC.txCount))
+		return hash, 0
+	}
 }
 
 func (appC *CounterAppContext) Commit() types.RetCode {