fix profiler errors if statbars are updated only

* Default to '??' (common Minetest notation) for unknown mods when loaded
* its related to a future implementation of statbars
* related to https://github.com/minetest/minetest/issues/8237

builtin/profiler/instrumentation.lua

@@ -88,7 +88,7 @@ local function instrument(def)
 	if not def or not def.func then
 		return
 	end
-	def.mod = def.mod or get_current_modname()
+	def.mod = def.mod or get_current_modname() or "??"
 	local modname = def.mod
 	local instrument_name = generate_name(def)
 	local func = def.func