sdap: Fix ldap_rfc_2307_fallback_to_local_users
    
    We wrongly tried to store empty
    user attributes instead of the
    local user attributes with
    ldap_rfc_2307_fallback_to_local_users
    set to true. This gave us bad
    initgroups results and caused
    segfaults.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3045
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit aa8ec3758d885d6ae4088174369d30f8493ec898)