Properly handle null and air items. Fixes BUKKIT-435 and BUKKIT-550

We'll probably want to implement an ItemStack.EMPTY and return that
instead of NULL in the near future.

src/main/java/org/bukkit/craftbukkit/CraftWorld.java

@@ -3,6 +3,8 @@ package org.bukkit.craftbukkit;
 import com.google.common.collect.MapMaker;
 import java.io.File;
 import java.util.Set;
+import org.apache.commons.lang.Validate;
+
 import org.bukkit.craftbukkit.entity.*;
 import org.bukkit.entity.*;
 import org.bukkit.entity.Entity;
@@ -299,6 +301,8 @@ public class CraftWorld implements World {
     }
 
     public org.bukkit.entity.Item dropItem(Location loc, ItemStack item) {
+        Validate.notNull(item, "Cannot drop a Null item.");
+        Validate.isTrue(item.getTypeId() != 0, "Cannot drop AIR.");
         CraftItemStack clone = new CraftItemStack(item);
         EntityItem entity = new EntityItem(world, loc.getX(), loc.getY(), loc.getZ(), clone.getHandle());
         entity.pickupDelay = 10;

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

@@ -27,7 +27,7 @@ public class CraftInventory implements org.bukkit.inventory.Inventory {
     }
 
     public ItemStack getItem(int index) {
-        return new CraftItemStack(getInventory().getItem(index));
+        return getInventory().getItem(index).id == 0 ? null : new CraftItemStack(getInventory().getItem(index));
     }
 
     public ItemStack[] getContents() {
@@ -59,7 +59,7 @@ public class CraftInventory implements org.bukkit.inventory.Inventory {
     }
 
     public void setItem(int index, ItemStack item) {
-        getInventory().setItem(index, (item == null ? null : CraftItemStack.createNMSItemStack(item)));
+        getInventory().setItem(index, ((item == null || item.getTypeId() == 0) ? null : CraftItemStack.createNMSItemStack(item)));
     }
 
     public boolean contains(int materialId) {