allow fill gui item to be air

instead of printing error on stats failure, just print a warning

src/main/java/sh/okx/rankup/Rankup.java

@@ -117,7 +117,7 @@ public class Rankup extends JavaPlugin {
       try {
         new Stats().init(this);
       } catch (IOException e) {
-        e.printStackTrace();
+        getLogger().warning("Could not connect to stats server");
       }
     }
   }

src/main/java/sh/okx/rankup/gui/Gui.java

@@ -70,6 +70,10 @@ public class Gui implements InventoryHolder {
       item = new ItemStack(material);
     }
 
+    if(item.getType() == Material.AIR && section.getName().equalsIgnoreCase("fill")) {
+      return item;
+    }
+
     ItemMeta meta = item.getItemMeta();
     if (section.contains("lore")) {
       meta.setLore(Arrays.stream(format(section.getString("lore"), player, oldRank, rank).split("\n"))