Fix node 0,0,0 being highlighted when enable_node_highlighting == false

Without this patch node 0,0,0 is highlighted when enable_node_highligting is false
There is a minor lighting issue remaining, however it seems to be related to a different bug (https://github.com/minetest/minetest/issues/1887)

See: https://forum.minetest.net/viewtopic.php?f=6&t=10549