Ignore BlockFertilizeEvent with empty list.

Plugins can clear this list without fully cancelling the event.
Fixes #1723

worldguard-bukkit/src/main/java/com/sk89q/worldguard/bukkit/listener/EventAbstractionListener.java

@@ -560,6 +560,7 @@ public class EventAbstractionListener extends AbstractListener {
 
     @EventHandler(ignoreCancelled = true)
     public void onBlockFertilize(BlockFertilizeEvent event) {
+        if (event.getBlocks().isEmpty()) return; 
         Cause cause = create(event.getPlayer(), event.getBlock());
         Events.fireToCancel(event, new PlaceBlockEvent(event, cause, event.getBlock().getWorld(), event.getBlocks()));
     }