Fix item drop issue on older spigot versions

bukkit/src/main/java/com/sk89q/worldedit/bukkit/BukkitPlayer.java

@@ -140,6 +140,7 @@ public class BukkitPlayer extends LocalPlayer {
                     public void run(Object value) {
                         for (Map.Entry<Integer, ItemStack> entry : overflow.entrySet()) {
                             ItemStack stack = entry.getValue();
+                            if (stack.getType() != Material.AIR && stack.getAmount() > 0) {
                                 Item dropped = player.getWorld().dropItem(player.getLocation(), stack);
                                 PlayerDropItemEvent event = new PlayerDropItemEvent(player, dropped);
                                 if (event.isCancelled()) {
@@ -147,6 +148,7 @@ public class BukkitPlayer extends LocalPlayer {
                                 }
                             }
                         }
+                    }
                 });
             }
         }