replace file system check with WF check

Given the assignment `$iVarId = $oMC->get( $sMCKey )`, `!isset( $iVarId )` would always evaluate to false. `$iVarId` can be `false` (cache expired) or a positive integer (cached). I need to pass a positive integer to `WikiFactory::getCityIDsFromVarValue()` in order to get reliable result. Otherwise I will be getting an empty array and `CreateWiki::wgUploadDirectoryExists()` will be giving false positives which could have serious consequences (multiple wikis having identical `wgUploadDirectory`). `WikiFactory::getVarIdByName()` returns a positive integer or `false`.