Fix invalid unlimited items preventing userdata to load

Invalid material names would previously return a null value
when deserializing. This raises an exception from within
Configurate because they use an EnumSet internally during
deserialization which doesn't support null elements.

Essentials/src/main/java/com/earth2me/essentials/config/serializers/MaterialTypeSerializer.java

@@ -17,7 +17,9 @@ public class MaterialTypeSerializer extends ScalarSerializer<Material> {
         if (obj instanceof String) {
             return Material.matchMaterial((String) obj);
         }
-        return null;
+
+        // Configurate will use an EnumSet to deserialize, which doesn't support null types. Default to air.
+        return Material.AIR;
     }
 
     @Override