[gtk/bilelmoussaoui/g-i] g-i: add missing nullable annotation

[Bilal Elmoussaoui <bilelmoussaoui src gnome org>]

commit ec58013b22f380cf992e9b39e66197b78249a703
Author: Bilal Elmoussaoui <bil elmoussaoui gmail com>
Date:   Thu Dec 30 16:52:53 2021 +0000

    g-i: add missing nullable annotation
    
    the constructor takes a nullable expression parameter, so should the getter be

 gtk/gtkexpression.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)
---
diff --git a/gtk/gtkexpression.c b/gtk/gtkexpression.c
index 8cd72f802f..14bea1111b 100644
--- a/gtk/gtkexpression.c
+++ b/gtk/gtkexpression.c
@@ -1,4 +1,3 @@
-
 /*
  * Copyright © 2019 Benjamin Otte
  *
@@ -1372,7 +1371,7 @@ gtk_property_expression_new_for_pspec (GtkExpression *expression,
  * Gets the expression specifying the object of
  * a property expression.
  *
- * Returns: (transfer none): the object expression
+ * Returns: (transfer none) (nullable): the object expression
  */
 GtkExpression *
 gtk_property_expression_get_expression (GtkExpression *expression)