[Statistics] fix behavioural tab without any project selection (#6882)

When no project is selected the behavioural tab should show results for all projects for all visits. instead it was just displaying an empty row see images.

modules/statistics/php/stats_behavioural.class.inc

@@ -128,6 +128,8 @@ class Stats_Behavioural extends \NDB_Form
                 $suproject_query ="AND s.SubprojectID IN ($subprojList)";
             }
             $Visits = \Utility::getExistingVisitLabels($currentProject);
+        } else {
+            $Visits = \Utility::getExistingVisitLabels();
         }
 
         $this->tpl_data['Visits'] = $Visits ?? '';