Fixed NPE on /party invite when used without a second argument

src/main/java/com/gmail/nossr50/party/commands/PartyInviteCommand.java

@@ -66,7 +66,7 @@ public class PartyInviteCommand implements CommandExecutor {
             return true;
 
         default:
-            player.sendMessage(LocaleLoader.getString("Commands.Usage.2", "party", "invite", "<" + LocaleLoader.getString("Commands.Usage.Player") + ">"));
+            sender.sendMessage(LocaleLoader.getString("Commands.Usage.2", "party", "invite", "<" + LocaleLoader.getString("Commands.Usage.Player") + ">"));
             return true;
         }
     }