finish tcp rendezvous keep alive logic following mqtt, but defined by

server so that it can be easily to be controlled at server side.

libs/hbb_common/src/tcp.rs

@@ -330,6 +330,9 @@ impl Encrypt {
     }
 
     pub fn dec(&mut self, bytes: &mut BytesMut) -> Result<(), Error> {
+        if bytes.is_empty() {
+            return Ok(());
+        }
         self.2 += 1;
         let nonce = FramedStream::get_nonce(self.2);
         match secretbox::open(bytes, &nonce, &self.0) {