[rygel] server: Fix missing nullable argument

[Jens Georg <jensgeorg src gnome org>]

commit 71228d54e872b8455d60cabadecd34d907c1412f
Author: Jens Georg <mail jensge org>
Date:   Tue Jun 9 18:03:57 2020 +0200

    server: Fix missing nullable argument
    
    Fixes #165

 src/librygel-server/rygel-searchable-container.vala | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)
---
diff --git a/src/librygel-server/rygel-searchable-container.vala 
b/src/librygel-server/rygel-searchable-container.vala
index 83076c1e..7e4a11ed 100644
--- a/src/librygel-server/rygel-searchable-container.vala
+++ b/src/librygel-server/rygel-searchable-container.vala
@@ -190,11 +190,11 @@ public interface Rygel.SearchableContainer : MediaContainer {
     }
 
     private async MediaObjects search_in_children
-                                        (SearchExpression expression,
-                                         MediaObjects     children,
-                                         uint             limit,
-                                         string           sort_criteria,
-                                         Cancellable?     cancellable)
+                                        (SearchExpression? expression,
+                                         MediaObjects      children,
+                                         uint              limit,
+                                         string            sort_criteria,
+                                         Cancellable?      cancellable)
                                         throws Error {
         var result = new MediaObjects ();