MDL-46833 mod_assign: Fix assignment view when available for group me…

…mbers only.

When assignment is "Available for group members only" and no users with
mod/assign:submit role permision is enrolled (students),
groups_filter_users_by_course_module_visible is causing error (as $users
array is empty). The patch resolves the issue.

lib/grouplib.php

@@ -860,6 +860,9 @@ function groups_get_activity_allowed_groups($cm,$userid=0) {
 function groups_filter_users_by_course_module_visible($cm, $users) {
     global $CFG, $DB;
 
+    if (empty($users)) {
+        return $users;
+    }
     if (empty($CFG->enablegroupmembersonly)) {
         return $users;
     }