Fix SWCommand with 0 args

Signed-off-by: Lixfel <git-5w3l@lixfel.de>

src/de/steamwar/command/AbstractSWCommand.java

@@ -102,7 +102,7 @@ public abstract class AbstractSWCommand<T> {
 
     protected final List<String> tabComplete(T sender, String alias, String[] args) throws IllegalArgumentException {
         initialize();
-        String string = args[args.length - 1].toLowerCase();
+        String string = args.length > 0 ? args[args.length - 1].toLowerCase() : "";
         return Stream.concat(commandList.stream(), helpCommandList.stream())
                 .filter(s -> !s.noTabComplete)
                 .map(s -> s.tabComplete(sender, args))