MDL-83457 mod_feedback: Fix get_incomplete_users early return type

Fix 'feedback_get_incomplete_users' early return when there are no users with
the 'mod/feedback:complete' capability from 'false' to an empty array. That
will follow the function signature.

mod/feedback/lib.php

@@ -964,7 +964,7 @@ function feedback_get_incomplete_users(cm_info $cm,
                                             $group,
                                             '',
                                             true)) {
-        return false;
+        return [];
     }
     // Filter users that are not in the correct group/grouping.
     $info = new \core_availability\info_module($cm);