Fix query for exercise categories in exerciseGrid() - refs BT#16036

main/exercise/exercise.class.php

@@ -8417,7 +8417,11 @@ class Exercise
         $categoryCondition = '';
         $categoryId = (int) $categoryId;
         if (api_get_configuration_value('allow_exercise_categories')) {
+            if (!empty($categoryId)) {
                 $categoryCondition = " AND exercise_category_id = $categoryId ";
+            } else {
+                $categoryCondition = " AND exercise_category_id IS NULL ";
+            }
         }
 
         $keywordCondition = '';