Add nil check for replace_with_item (#28)

Needed because str:format in Lua5.1 will not interpret nil values, whereas LuaJIT will interpret them.

init.lua

@@ -423,8 +423,13 @@ function minetest.do_item_eat(hp_change, replace_with_item, itemstack, player, p
 	end
 
 	local itemname = itemstack:get_name()
-	stamina.log("action", "%s eats %s for %s stamina, replace with %s",
+	if replace_with_item then
+		stamina.log("action", "%s eats %s for %s stamina, replace with %s",
 			player:get_player_name(), itemname, hp_change, replace_with_item)
+	else
+		stamina.log("action", "%s eats %s for %s stamina",
+			player:get_player_name(), itemname, hp_change)
+	end
 	minetest.sound_play("stamina_eat", {to_player = player:get_player_name(), gain = 0.7})
 
 	if hp_change > 0 then