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>