Don't g_object_unref a NULL object for event variant
Don't g_object_unref a NULL object for event variant

If we get an event with what appears to be an accessible as the variant
but we do not succeed in marshalling the accessible (possibly because it
is the null path), do not unref the null object. Stops a glib warning.

GNOME/at-spi2-core,GNOME/at-spi2-core,GNOME/at-spi2-core