Minor - fix php warning

main/gradebook/lib/be/attendancelink.class.php

@@ -153,7 +153,7 @@ class AttendanceLink extends AbstractLink
             }
 
             if ($rescount == 0) {
-                return null;
+                return [null, null];
             } else {
                 switch ($type) {
                     case 'best':

main/gradebook/lib/be/exerciselink.class.php

@@ -203,7 +203,7 @@ class ExerciseLink extends AbstractLink
                         return $result;
                         break;
                     case 'ranking':
-                        return '';
+                        return [null, null];
                         /*
                         $newList = [];
                         $ranking = AbstractLink::getCurrentUserRanking($stud_id, $link->getUserScoreList());

main/gradebook/lib/be/forumthreadlink.class.php

@@ -241,7 +241,7 @@ class ForumThreadLink extends AbstractLink
             }
 
             if ($counter == 0) {
-                return null;
+                return [null, null];
             } else {
                 switch ($type) {
                     case 'best':

main/gradebook/lib/be/learnpathlink.class.php

@@ -129,7 +129,7 @@ class LearnpathLink extends AbstractLink
             }
 
             if ($rescount == 0) {
-                return null;
+                return [null, null];
             } else {
                 switch ($type) {
                     case 'best':

main/gradebook/lib/be/surveylink.class.php

@@ -179,7 +179,7 @@ class SurveyLink extends AbstractLink
             $sum = $sum / $max_score;
 
             if ($rescount == 0) {
-                return null;
+                return [null, null];
             }
 
             switch ($type) {