Brand: ItemData.Get already refuses an unset ref; drop the duplicate IsSet guard

Stryker's one survivor in the first sweep was the guard's `true ?` mutant: the
cache's Get(ref) extension already returns null for an unset reference, so the
guard was a second path. Adds the removed-product fact the spec names.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assets/Scripts/ServerShared/ItemManager.cs

@@ -216,7 +216,7 @@ public class ItemManager
         };
         if (!maker.IsSet()) return (null, null);
 
-        var product = lot.Product.IsSet() ? ItemData.Get(lot.Product) : null;
+        var product = ItemData.Get(lot.Product);
         return (ItemData.Get(maker), product);
     }