fix crash caused by unknown nodes

[a-tour-ist]

there are missing nil checks for minetest.registered_nodes[something]

If you look closely, you can see that there is one place left:

castle_gates/gate_functions.lua

Lines 165 to 166 in 8449b54

	local test_node_def = minetest.registered_nodes[test_node.name]
	if test_node_def.buildable_to then

As far as I can see, this one should already be covered by the check on L217.
But I can add one for good measure if wanted.