Falling: Fix error caused by missing param2

Falling nodes that were spawned prior the recent falling node changes did not require param2.
Default to param2 = 0 when none is found in the node data.

builtin/game/falling.lua

@@ -52,6 +52,7 @@ core.register_entity(":__builtin:falling_node", {
 	floats = false,
 
 	set_node = function(self, node, meta)
+		node.param2 = node.param2 or 0
 		self.node = node
 		meta = meta or {}
 		if type(meta.to_table) == "function" then