Add empty data to SSH_MSG_IGNORE messages (#974)

As required by RFC 4253 Section 11.2, and RFC 4251 Section 5

Co-authored-by: Jeroen van Erp <jeroen@hierynomus.com>

src/main/java/net/schmizz/keepalive/Heartbeater.java

@@ -29,6 +29,6 @@ final class Heartbeater
 
     @Override
     protected void doKeepAlive() throws TransportException {
-        conn.getTransport().write(new SSHPacket(Message.IGNORE));
+        conn.getTransport().write(new SSHPacket(Message.IGNORE).putString(""));
     }
 }