wire: only write message payload if there actually is one

There are certain messages that won't have a payload, e.g., verack,
causing an unnecessary write of 0 bytes.

wire/message.go

@@ -321,9 +321,13 @@ func WriteMessageWithEncodingN(w io.Writer, msg Message, pver uint32,
 		return totalBytes, err
 	}
 
-	// Write payload.
-	n, err = w.Write(payload)
-	totalBytes += n
+	// Only write the payload if there is one, e.g., verack messages don't
+	// have one.
+	if len(payload) > 0 {
+		n, err = w.Write(payload)
+		totalBytes += n
+	}
+
 	return totalBytes, err
 }