[glib] gio: Mark g_task_get_source_object as nullable

[Rico Tzschichholz <ricotz src gnome org>]

commit 1fb56be6abbddee62ac09b0cf333d6f2227ba496
Author: Rico Tzschichholz <ricotz ubuntu com>
Date:   Wed Oct 11 17:14:11 2017 +0200

    gio: Mark g_task_get_source_object as nullable
    
    In conjunction with ca4fe5942afa5d24f2c99f12d41598cf04b7718f
    
    Reviewed-by: Philip Withnall <withnall endlessm com>

 gio/gtask.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/gio/gtask.c b/gio/gtask.c
index 7eb0185..9a0c353 100644
--- a/gio/gtask.c
+++ b/gio/gtask.c
@@ -978,7 +978,7 @@ g_task_set_source_tag (GTask    *task,
  * Gets the source object from @task. Like
  * g_async_result_get_source_object(), but does not ref the object.
  *
- * Returns: (transfer none) (type GObject): @task's source object, or %NULL
+ * Returns: (transfer none) (nullable) (type GObject): @task's source object, or %NULL
  *
  * Since: 2.36
  */