SPIGOT-2772, SPIGOT-2780: Improve empty ItemStack handling

src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java

@@ -1016,7 +1016,7 @@ public class CraftEventFactory {
 
     public static EntityBreedEvent callEntityBreedEvent(EntityLiving child, EntityLiving mother, EntityLiving father, EntityLiving breeder, ItemStack bredWith, int experience) {
         org.bukkit.entity.LivingEntity breederEntity = (LivingEntity)(breeder == null ? null : breeder.getBukkitEntity());
-        CraftItemStack bredWithStack = bredWith == null ? null : CraftItemStack.asCraftMirror(bredWith).clone();
+        CraftItemStack bredWithStack = (bredWith.isEmpty()) ? null : CraftItemStack.asCraftMirror(bredWith).clone();
 
         EntityBreedEvent event = new EntityBreedEvent((LivingEntity) child.getBukkitEntity(), (LivingEntity) mother.getBukkitEntity(), (LivingEntity) father.getBukkitEntity(), breederEntity, bredWithStack, experience);
         child.world.getServer().getPluginManager().callEvent(event);

src/main/java/org/bukkit/craftbukkit/inventory/CraftInventory.java

@@ -46,7 +46,7 @@ public class CraftInventory implements Inventory {
 
     public ItemStack getItem(int index) {
         net.minecraft.server.ItemStack item = getInventory().getItem(index);
-        return item == null ? null : CraftItemStack.asCraftMirror(item);
+        return CraftItemStack.asCraftMirror(item);
     }
 
     @Override
@@ -65,7 +65,7 @@ public class CraftInventory implements Inventory {
 
         int size = Math.min(items.length, mcItems.size());
         for (int i = 0; i < size; i++) {
-            items[i] = (mcItems.get(i) == net.minecraft.server.ItemStack.a) ? null : CraftItemStack.asCraftMirror(mcItems.get(i));
+            items[i] = CraftItemStack.asCraftMirror(mcItems.get(i));
         }
         return items;
     }

src/main/java/org/bukkit/craftbukkit/inventory/CraftInventoryAnvil.java

@@ -28,19 +28,19 @@ public class CraftInventoryAnvil extends CraftInventory implements AnvilInventor
     public ItemStack getItem(int slot) {
         if (slot < getIngredientsInventory().getSize()) {
             net.minecraft.server.ItemStack item = getIngredientsInventory().getItem(slot);
-            return item == null ? null : CraftItemStack.asCraftMirror(item);
+            return CraftItemStack.asCraftMirror(item);
         } else {
             net.minecraft.server.ItemStack item = getResultInventory().getItem(slot - getIngredientsInventory().getSize());
-            return item == null ? null : CraftItemStack.asCraftMirror(item);
+            return CraftItemStack.asCraftMirror(item);
         }
     }
 
     @Override
     public void setItem(int index, ItemStack item) {
         if (index < getIngredientsInventory().getSize()) {
-            getIngredientsInventory().setItem(index, (item == null ? null : CraftItemStack.asNMSCopy(item)));
+            getIngredientsInventory().setItem(index, CraftItemStack.asNMSCopy(item));
         } else {
-            getResultInventory().setItem((index - getIngredientsInventory().getSize()), (item == null ? null : CraftItemStack.asNMSCopy(item)));
+            getResultInventory().setItem((index - getIngredientsInventory().getSize()), CraftItemStack.asNMSCopy(item));
         }
     }
 

src/main/java/org/bukkit/craftbukkit/inventory/CraftInventoryCrafting.java

@@ -70,19 +70,19 @@ public class CraftInventoryCrafting extends CraftInventory implements CraftingIn
     public CraftItemStack getItem(int index) {
         if (index < getResultInventory().getSize()) {
             net.minecraft.server.ItemStack item = getResultInventory().getItem(index);
-            return item == null ? null : CraftItemStack.asCraftMirror(item);
+            return CraftItemStack.asCraftMirror(item);
         } else {
             net.minecraft.server.ItemStack item = getMatrixInventory().getItem(index - getResultInventory().getSize());
-            return item == null ? null : CraftItemStack.asCraftMirror(item);
+            return CraftItemStack.asCraftMirror(item);
         }
     }
 
     @Override
     public void setItem(int index, ItemStack item) {
         if (index < getResultInventory().getSize()) {
-            getResultInventory().setItem(index, (item == null ? null : CraftItemStack.asNMSCopy(item)));
+            getResultInventory().setItem(index, CraftItemStack.asNMSCopy(item));
         } else {
-            getMatrixInventory().setItem((index - getResultInventory().getSize()), (item == null ? null : CraftItemStack.asNMSCopy(item)));
+            getMatrixInventory().setItem((index - getResultInventory().getSize()), CraftItemStack.asNMSCopy(item));
         }
     }
 
@@ -99,8 +99,7 @@ public class CraftInventoryCrafting extends CraftInventory implements CraftingIn
 
     public ItemStack getResult() {
         net.minecraft.server.ItemStack item = getResultInventory().getItem(0);
-        if(item != null) return CraftItemStack.asCraftMirror(item);
-        return null;
+        return CraftItemStack.asCraftMirror(item);
     }
 
     public void setMatrix(ItemStack[] contents) {

src/main/java/org/bukkit/craftbukkit/inventory/CraftItemStack.java

@@ -57,7 +57,7 @@ public final class CraftItemStack extends ItemStack {
      * Copies the NMS stack to return as a strictly-Bukkit stack
      */
     public static ItemStack asBukkitCopy(net.minecraft.server.ItemStack original) {
-        if (original == net.minecraft.server.ItemStack.a) {
+        if (original.isEmpty()) {
             return new ItemStack(Material.AIR);
         }
         ItemStack stack = new ItemStack(CraftMagicNumbers.getMaterial(original.getItem()), original.getCount(), (short) original.getData());
@@ -68,7 +68,7 @@ public final class CraftItemStack extends ItemStack {
     }
 
     public static CraftItemStack asCraftMirror(net.minecraft.server.ItemStack original) {
-        return new CraftItemStack(original);
+        return (original.isEmpty()) ? null : new CraftItemStack(original);
     }
 
     public static CraftItemStack asCraftCopy(ItemStack original) {