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-source-to-PlayerExpChangeEvent.patch

@@ -37,7 +37,7 @@ index f37491d7..30882559 100644
           exp = expAmount;
      }
  
-     /**
++    /**
 +     * Get the source that provided the experience.
 +     *
 +     * @return The source of the experience
@@ -48,8 +48,7 @@ index f37491d7..30882559 100644
 +    }
 +    // Paper end
 +
-+    /**
+     /**
       * Get the amount of experience the player will receive
       *
-      * @return The amount of experience
 --