Fixed a crash resulting from uninstalling and reinstalling

After uninstalling, all the addon folders are removed, and so the cache never gets initialized. This results in a crash in a python socket callback when it tries to take None as an argument.
Left in the conditional in case it does something I don't understand, but sanity-checks for existence and just returns [] if None.
ok, I will this, but in a bit different way: return self._cache.get("mods", []). <- the second parameter is the default value, for the case if the key is missing in the dictionary ;)