Fix: only treat command as signed if it has signatures

The Vanilla client will send 'last seen' even if the message is not signed, so this is much stricter than it needs to be. This should allow commands without any signed arguments to be modified/consumed by the proxy.

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/chat/session/SessionPlayerCommandPacket.java

@@ -65,8 +65,7 @@ public class SessionPlayerCommandPacket implements MinecraftPacket {
   }
 
   public boolean isSigned() {
-    if (salt == 0) return false;
-    return !lastSeenMessages.isEmpty() || !argumentSignatures.isEmpty();
+    return !argumentSignatures.isEmpty();
   }
 
   @Override