[gtk] label: Add missing nullable annotation

[Timm Bäder <baedert src gnome org>]

commit 5680393f9d966e15b3a3390695514462f7a9731d
Author: Timm Bäder <mail baedert org>
Date:   Sat Jan 12 08:03:47 2019 +0100

    label: Add missing nullable annotation

 gtk/gtklabel.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtklabel.c b/gtk/gtklabel.c
index 1d4223aada..57db38485b 100644
--- a/gtk/gtklabel.c
+++ b/gtk/gtklabel.c
@@ -6254,8 +6254,8 @@ gtk_label_get_current_link (GtkLabel *label)
  * This function is intended for use in a #GtkLabel::activate-link handler
  * or for use in a #GtkWidget::query-tooltip handler.
  *
- * Returns: the currently active URI. The string is owned by GTK+ and must
- *   not be freed or modified.
+ * Returns: (nullable): the currently active URI or %NULL if there is none.
+ *   The string is owned by GTK+ and must not be freed or modified.
  */
 const gchar *
 gtk_label_get_current_uri (GtkLabel *label)