confdb: use proper timestamp if sssd.conf is missing
If sssd.conf is missing the timestamp is uninitialized and as a result
the lastUpdate attribute in config.ldb will contain some random binary
value.
This patch initializes the timestamp to "1".
Resolves: https://pagure.io/SSSD/sssd/issue/4178
Reviewed-by: Alexey Tikhonov <atikhonov@redhat.com>