SYSDB: Return ERR_NO_TS when there's no timestamp cache present
    
    This change affects sysdb_search_ts_{users,groups} functions and is
    mainly needed in order to avoid breaking our current tests due to the
    changes planned for fixing https://pagure.io/SSSD/sssd/issue/3369.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3369
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 01c6bb9b47401f9f14c4cfe5c5f03fce2e63629b)