Don't try to check if other reactor chambers are powered if Nuclear Reactor wasn't initialized

Fixes #7

compat/mesecons.lua

@@ -93,11 +93,13 @@ override={
 				if meta:get_int("meseconPowered")==1 then
 					return
 				end
-				local chambers=minetest.deserialize(meta:get_string("chambers"))
-				for _,chamber in ipairs(chambers) do
-					local chamberMeta=minetest.get_meta(chamber)
-					if chamberMeta:get_int("meseconPowered")==1 then
-						return
+				if meta:contains("chambers") then
+					local chambers=minetest.deserialize(meta:get_string("chambers"))
+					for _,chamber in ipairs(chambers) do
+						local chamberMeta=minetest.get_meta(chamber)
+						if chamberMeta:get_int("meseconPowered")==1 then
+							return
+						end
 					end
 				end