Fix 0-RTT write zero

src/client.rs

@@ -129,8 +129,10 @@ where
                         Err(err) => return Poll::Ready(Err(err))
                     };
                     data.extend_from_slice(&buf[..len]);
+                    if len != 0 {
                         return Poll::Ready(Ok(len));
                     }
+                }
 
                 // complete handshake
                 while stream.session.is_handshaking() {