g-i: add missing nullable annotation

the constructor takes a nullable expression parameter, so should the getter be

diff --git a/gtk/gtkexpression.c b/gtk/gtkexpression.c
index 8cd72f802fe9d9cf3d30548cd7acf97abb7b0819..14bea1111b0af02a0ce544edffc8dc01c17d37e2 100644 (file)
--- 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)