shellDBus: don't consider a zero-index monitor invalid

Currently, shellDBus only uses the passed in monitor index if it's
strictly > 0. A zero-index monitor is a valid one though, so don't
restrict this to strictly positive indices.

https://bugzilla.gnome.org/show_bug.cgi?id=740074

js/ui/shellDBus.js

@@ -134,7 +134,7 @@ const GnomeShell = new Lang.Class({
             params[param] = params[param].deep_unpack();
 
         let monitorIndex = -1;
-        if (params['monitor'])
+        if (params['monitor'] >= 0)
             monitorIndex = params['monitor'];
 
         let icon = null;