Deserialization: Restore backwards compat (#12519)

builtin/common/serialize.lua

@@ -188,6 +188,16 @@ local function dummy_func() end
 local nan = (0/0)^1 -- +nan
 
 function core.deserialize(str, safe)
+	-- Backwards compatibility
+	if str == nil then
+		core.log("deprecated", "minetest.deserialize called with nil (expected string).")
+		return nil, "Invalid type: Expected a string, got nil"
+	end
+	local t = type(str)
+	if t ~= "string" then
+		error(("minetest.deserialize called with %s (expected string)."):format(t))
+	end
+
 	local func, err = loadstring(str)
 	if not func then return nil, err end