session: Don't accept empty string as saved language
    
    accountsservice currently returns the empty string for Language (and
    other string keys) when they're unset, instead of NULL.  GDM should be
    defensive against this.
    
    This way we get a proper fallback to the system's default locale.
    Fixes setting of $LANG in user session in gnome-ostree buildmaster.
    
    See also:
    http://lists.freedesktop.org/archives/systemd-devel/2013-January/007945.html
    for a discussion of the current state of $LANG, pam, systemd, gdm, and
    accountsservice.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=699261