Fix error on unknown node

actions/pathfinder.lua

@@ -103,7 +103,7 @@ local function is_good_node(node, exceptions)
       break
     end
   end
-  if not minetest.registered_nodes[node.name].walkable then
+  if node.name ~= nil and not minetest.registered_nodes[node.name].walkable then
     return pathfinder.node_types.walkable
   elseif is_openable then
     return pathfinder.node_types.openable
@@ -250,4 +250,4 @@ function pathfinder.get_path(map, path_nodes)
     --print(('Step: %d - x: %d - y: %d'):format(count, node:getX(), node:getY()))
   end
   return result
-end
+end