[rygel] tracker: Prepare for new vala

[Jens Georg <jensgeorg src gnome org>]

commit 335c94435f63c82b53b6c5ffc2e2a7eac04b3444
Author: Jens Georg <mail jensge org>
Date:   Mon Apr 6 09:38:52 2020 +0200

    tracker: Prepare for new vala
    
    Upcoming Vala will have corrected bindings; since the fix already works
    with older vala versions, just do it anyway.

 src/plugins/tracker/rygel-tracker-category-all-container.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/plugins/tracker/rygel-tracker-category-all-container.vala 
b/src/plugins/tracker/rygel-tracker-category-all-container.vala
index 460c1d17..3845b7fb 100644
--- a/src/plugins/tracker/rygel-tracker-category-all-container.vala
+++ b/src/plugins/tracker/rygel-tracker-category-all-container.vala
@@ -127,7 +127,7 @@ public class Rygel.Tracker.CategoryAllContainer : SearchContainer,
     }
 
     private void on_graph_updated (DBusConnection connection,
-                                   string         sender,
+                                   string?        sender,
                                    string         object_path,
                                    string         interface_name,
                                    string         signal_path,