Remove null values in TileEntity tick list

Because who wouldn't want to tick a null tile entity right?
Fixes GH-851

Spigot-API-Patches/Add-async-chunk-load-API.patch

@@ -5,13 +5,14 @@ Subject: [PATCH] Add async chunk load API
 
 
 diff --git a/src/main/java/org/bukkit/World.java b/src/main/java/org/bukkit/World.java
-index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
+index 67b0d516..21b2733f 100644
 --- a/src/main/java/org/bukkit/World.java
 +++ b/src/main/java/org/bukkit/World.java
 @@ -0,0 +0,0 @@ public interface World extends PluginMessageRecipient, Metadatable {
+      */
      public Chunk getChunkAt(Block block);
  
-     /**
++    /**
 +     * Used by {@link World#getChunkAtAsync(Location,ChunkLoadCallback)} methods
 +     * to request a {@link Chunk} to be loaded, with this callback receiving
 +     * the chunk when it is finished.
@@ -83,8 +84,7 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +     */
 +    public void getChunkAtAsync(Block block, ChunkLoadCallback cb);
 +
-+    /**
+     /**
       * Checks if the specified {@link Chunk} is loaded
       *
-      * @param chunk The chunk to check
 --