[gnome-shell] portalHelper: Fix runtime warning

[Bastien Nocera <hadess src gnome org>]

commit 98cdd4454323ca04f49a1c4f46f404fd2b6a3efd
Author: Bastien Nocera <hadess hadess net>
Date:   Mon Feb 13 11:10:19 2017 +0100

    portalHelper: Fix runtime warning
    
    When there is no URI, then the title should be empty, not null:
    Gjs-WARNING **: JS ERROR: Error: Argument 'str' (type utf8) may not be null
    
    https://bugzilla.gnome.org/show_bug.cgi?id=778552

 js/portalHelper/main.js |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/js/portalHelper/main.js b/js/portalHelper/main.js
index ccbe77b..6070af2 100644
--- a/js/portalHelper/main.js
+++ b/js/portalHelper/main.js
@@ -176,7 +176,7 @@ const PortalWindow = new Lang.Class({
         if (uri)
             this._headerBar.setSubtitle(GLib.uri_unescape_string(uri, null));
         else
-            this._headerBar.setSubtitle(null);
+            this._headerBar.setSubtitle('');
     },
 
     refresh: function() {