If you listen to a bucket event and try to set the resulting item stack in the player's hand, there is no effect. It will update the ItemStack at the API level, meaning you can retrieve the new ItemStack with getItemStack(), but the actual in-game item will not be changed. They will always receive an empty bucket.

The resulting event handler illustrates this behavior on the latest Spigot version:

@EventHandler
public void onPlayerBucketEmpty(PlayerBucketEmptyEvent event) {
    event.setItemStack(new ItemStack(Material.DIRT, 1));  
}