Update AbstractBase.php

It seems this solved issue #31

According to the docs, "onNewVisit" should return false if it is not supposed to update any information: https://developer.piwik.org/api-reference/Piwik/Plugin/Dimension/VisitDimension#onnewvisit

Columns/AbstractBase.php

@@ -31,6 +31,6 @@ protected function getLocationIntranetColumn(array $userInfo, $locationKey)
             return $result[$locationKey];
         }
 
-        return;
+        return false;
     }
 }