Revert "Optimize ItemStack.isEmpty()"

This reverts commit 257c591391.

See a bug

Spigot-Server-Patches/0188-Optimize-ItemStack.isEmpty.patch

@@ -1,31 +0,0 @@
-From 3ec4007a5c8e90d22c60d308a21d858e0cfcccc8 Mon Sep 17 00:00:00 2001
-From: Aikar <aikar@aikar.co>
-Date: Wed, 21 Dec 2016 03:48:29 -0500
-Subject: [PATCH] Optimize ItemStack.isEmpty()
-
-Remove hashMap lookup every check, simplify code to remove ternary
-
-diff --git a/src/main/java/net/minecraft/server/ItemStack.java b/src/main/java/net/minecraft/server/ItemStack.java
-index c8694e8b9..23c975891 100644
---- a/src/main/java/net/minecraft/server/ItemStack.java
-+++ b/src/main/java/net/minecraft/server/ItemStack.java
-@@ -112,9 +112,15 @@ public final class ItemStack {
-         this.F();
-     }
- 
-+    // Paper start - optimize isEmpty
-+    private static Item airItem;
-     public boolean isEmpty() {
--        return this == ItemStack.a ? true : (this.item != null && this.item != Item.getItemOf(Blocks.AIR) ? (this.count <= 0 ? true : this.damage < -32768 || this.damage > '\uffff') : true);
-+        if (airItem == null) {
-+            airItem = Item.getItemOf(Blocks.AIR);
-+        }
-+        return this == ItemStack.a || this.item == null || this.item != airItem || this.count <= 0 || this.damage < -32768 || this.damage > '\uffff';
-     }
-+    // Paper end
- 
-     public static void a(DataConverterManager dataconvertermanager) {
-         dataconvertermanager.a(DataConverterTypes.ITEM_INSTANCE, (DataInspector) (new DataInspectorBlockEntity()));
--- 
-2.11.0
-