Treat empty Glib::spawn*() working dir as unset

* glib/src/spawn.ccg (Glib::spawn_async_with_pipes): If the
working_directory argument is the empty string, pass a 0 pointer
to the GLib C API to make it inherit the parent's working directory.
This is fine as the empty string is not a valid directory name, and
our documentation already says that it will be interpreted that way.
(Glib::spawn_async): ditto,
(Glib::spawn_sync): ditto.