Update Wolowitz.pm

As it seems, not all user-agents populate the ->accept_language value. Robots? Probably. Haven't poked around in the logs...

Anyway, I've found an ugly warnings about "unitialized value in substitution" when $lang is empty in my error logs.

So let's return empty from _detect_lang_from_browser for such cases. Which should let the module->L::W fall-back onto the default lang, right?