fix crash if detected light at a node is nil

nature_classic/global_function.lua

@@ -43,7 +43,7 @@ end
 
 function nature:grow_node(pos, nodename)
     if pos ~= nil then
-        local light_enough = minetest.get_node_light(pos, nil)
+        local light_enough = (minetest.get_node_light(pos, nil) or 0)
                 >= nature.minimum_growth_light 
 
         if is_not_young(pos) and light_enough then