[gtk+] gdkwindow-x11: always set _GTK_THEME_VARIANT property

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 8eb261988869608604c78ed90de5579beb4ef2b0
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Thu Feb 4 11:52:07 2016 +0200

    gdkwindow-x11: always set _GTK_THEME_VARIANT property
    
    Set _GTK_THEME_VARIANT to empty string when default theme variant
    is used. This will allow to understand whether _GTK_THEME_VARIANT
    is not supported or default variant is requested.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=761476

 gdk/x11/gdkwindow-x11.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/gdk/x11/gdkwindow-x11.c b/gdk/x11/gdkwindow-x11.c
index ffe9776..8544de8 100644
--- a/gdk/x11/gdkwindow-x11.c
+++ b/gdk/x11/gdkwindow-x11.c
@@ -3728,7 +3728,8 @@ void
 gdk_x11_window_set_theme_variant (GdkWindow *window,
                                   char      *variant)
 {
-  gdk_x11_window_set_utf8_property (window, "_GTK_THEME_VARIANT", variant);
+  gdk_x11_window_set_utf8_property (window, "_GTK_THEME_VARIANT",
+                                    variant ? variant : "");
 }
 
 #define GDK_SELECTION_MAX_SIZE(display)                                 \