[gnome-boxes] installer-media: Correct null check

[Zeeshan Ali Khattak <zeeshanak src gnome org>]

commit b78ad22417dbf2da3672e2f2fb84b3fc0d3f94d2
Author: Zeeshan Ali (Khattak) <zeeshanak gnome org>
Date:   Wed Apr 3 22:30:18 2013 +0300

    installer-media: Correct null check
    
    This fixes build of git master.

 src/installer-media.vala |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/installer-media.vala b/src/installer-media.vala
index 7791085..471e87f 100644
--- a/src/installer-media.vala
+++ b/src/installer-media.vala
@@ -191,7 +191,7 @@ private class Boxes.InstallerMedia : GLib.Object {
 
     private string? get_decoded_udev_property (GUdev.Device device, string property_name) {
         var encoded = device.get_property (property_name);
-        if (!encoded)
+        if (encoded == null)
             return null;
 
         var decoded = "";