The heldItem parameter was removed from the method in 1.11, since you can get it from the EntityPlayer using EntityLivingBase#getHeldItem (EntityPlayer extends EntityLivingBase).
EntityLivingBase#getActiveItemStack only returns a non-empty ItemStack when the entity is actively using an item (e.g. blocking with a shield, drawing a bow).
ItemStacks can no longer be null in 1.11+, the default value is now the empty ItemStack. Use ItemStack#isEmpty to check if an ItemStack is empty. The ItemStack.EMPTY field contains an ItemStack that's always empty.