MousePerspective: allow shutdown when no device

If _device was not set yet then we will get an exception
when accessing its 'profiles' attribute. GObject.Property
does not support exceptions and we end up returning False.
This caused Piper to think that there is a dirty profile and
that warns the user about unapplied changes.

piper/mouseperspective.py

@@ -67,6 +67,9 @@ class MousePerspective(Gtk.Overlay):
 
     @GObject.Property
     def can_shutdown(self):
+        if self._device is None:
+            return True
+
         """Whether this perspective can safely shutdown."""
         for profile in self._device.profiles:
             if profile.dirty: