[gtk/bilelmoussaoui/g-i] gtk: mark gtk_lock_button_get_permission as nullable

[Bilal Elmoussaoui <bilelmoussaoui src gnome org>]

commit dcbf3f8879d197fa12c0a1be2ec711f66648e2d9
Author: Bilal Elmoussaoui <bil elmoussaoui gmail com>
Date:   Sat Jan 1 18:17:31 2022 +0100

    gtk: mark gtk_lock_button_get_permission as nullable
    
    like the constructor/setter

 gtk/gtklockbutton.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtk/gtklockbutton.c b/gtk/gtklockbutton.c
index 5862e866d8..dda8cc9147 100644
--- a/gtk/gtklockbutton.c
+++ b/gtk/gtklockbutton.c
@@ -535,7 +535,7 @@ gtk_lock_button_new (GPermission *permission)
  *
  * Obtains the `GPermission` object that controls @button.
  *
- * Returns: (transfer none): the `GPermission` of @button
+ * Returns: (transfer none) (nullable): the `GPermission` of @button
  */
 GPermission *
 gtk_lock_button_get_permission (GtkLockButton *button)