avoid undeclared global variable access

remove warning 
Undeclared global variable "hw_utils" accessed at ...ds/ambience/weather_pack/utils.lua:9

utils.lua

@@ -6,7 +6,7 @@
 -- Credits: xeranas
 ---------------------------------------
 
-if hw_utils == nil then
+if not minetest.global_exists("hw_utils") then
 	hw_utils = {}
 end