Minor - fix PHP warning

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

@@ -559,7 +559,7 @@ abstract class AbstractLink implements GradebookItem
 
     public function get_all_links()
     {
-        return null;
+        return [];
     }
 
     public function add_linked_data()

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

@@ -66,7 +66,7 @@ class AttendanceLink extends AbstractLink
                 $cats[] = [$data['id'], $data['name']];
             }
         }
-        $my_cats = isset($cats) ? $cats : null;
+        $my_cats = isset($cats) ? $cats : [];
 
         return $my_cats;
     }

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

@@ -117,7 +117,7 @@ class ForumThreadLink extends AbstractLink
                 $cats[] = [$data['thread_id'], $data['thread_title']];
             }
         }
-        $my_cats = isset($cats) ? $cats : null;
+        $my_cats = isset($cats) ? $cats : [];
 
         return $my_cats;
     }

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

@@ -90,7 +90,7 @@ class SurveyLink extends AbstractLink
             ];
         }
 
-        return isset($links) ? $links : null;
+        return isset($links) ? $links : [];
     }
 
     /**