Fix to nbt placeholder and new nbt return value of "" instead of null

src/me/rockyhawk/commandpanels/classresources/placeholders/Placeholders.java

@@ -147,7 +147,7 @@ public class Placeholders {
                 String slot_key = identifier.replace("nbt-", "");
                 String value;
                 value = plugin.nbt.getNBT(p.getOpenInventory().getTopInventory().getItem((int)Double.parseDouble(slot_key.split(":")[0])),slot_key.split(":")[1]);
-                if(value == null){
+                if(value.isEmpty()){
                     value = "empty";
                 }
                 return value;