Return null when no public kit IDs for tab completion

- Store public kit IDs before returning tab-complete results
- Return `null` instead of an empty list when no public kits exist

Author: rossnoah

src/main/java/dev/noah/perplayerkit/commands/admin/SavePublicKitCommand.java

@@ -93,7 +93,8 @@ public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command
     @Override
     public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
         if (args.length == 1) {
-            return KitManager.get().getPublicKitList().stream().map(kit -> kit.id).toList();
+            List<String> ids = KitManager.get().getPublicKitList().stream().map(kit -> kit.id).toList();
+            return ids.isEmpty() ? null : ids;
         }
 
         return null;