sss_cache: Do not fail for missing domains
    
    The conf.db needn't exist(sssd has never been started) and in such situation
    sss_cache failed when trying to invalidate all entries.
    
    There is nothing to invalidate and therefore we are already in state
    which we want to achieve with calling sss_cache.
    No reason to fail.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3919
    
    Merges: https://pagure.io/SSSD/sssd/pull-request/3926
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>