[polari] mainWindow: Don't use null as default value for MainWindow:subtitle

[Florian Müllner <fmuellner src gnome org>]

commit b693fd8974d96f12f8bdf44c6f9e16a41e9cbde8
Author: Florian Müllner <fmuellner gnome org>
Date:   Thu Feb 11 17:56:33 2016 +0100

    mainWindow: Don't use null as default value for MainWindow:subtitle
    
    This fails with slightly older versions of g-i/gjs, and using the empty
    string makes more sense anyway ...

 src/mainWindow.js |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/mainWindow.js b/src/mainWindow.js
index 1149757..136ba81 100644
--- a/src/mainWindow.js
+++ b/src/mainWindow.js
@@ -107,7 +107,7 @@ const MainWindow = new Lang.Class({
                                            'subtitle',
                                            'subtitle',
                                            GObject.ParamFlags.READABLE,
-                                           null),
+                                           ''),
         'subtitle-visible': GObject.ParamSpec.boolean('subtitle-visible',
                                                       'subtitle-visible',
                                                       'subtitle-visible',