Fix error when item isn't provided to core.hud_replace_builtin

[rubenwardy]

Context: https://forum.minetest.net/viewtopic.php?f=18&t=28131

In the wild: https://github.com/CasimirKaPazi/Voxelgarden/blob/b0893e89b08cc8702f21805c7316599f2dc98983/mods/default/gui.lua#L2

To do

This PR is Ready for Review.

How to test

local health_bar_definition =
{
	hud_elem_type = "statbar",
	position = { x=0.5, y=1 },
	text = "heart.png",
	number = 20,
	direction = 0,
	size = {x=16, y=16},
	offset = {x=(-9*24)-12, y=-(48+24+8)},
}

minetest.hud_replace_builtin("health", health_bar_definition)