osd/PeeringState: fix wrong history of merge target

osd/PeeringState: fix wrong history of merge target

The merge target could be a stray pg that has never got the chance
to finish a complete peering cycle (we never purge strays before merge
is done) and hence leaving a zeroed last_epoch_started in the info.

In case the merge target incorrectly absorbs an invalid history in,
e.g., from an actual placeholder merge source pg, use

    history.epoch_created == 0

to identify if the merge target is truly a placeholder instead.

Fixes: https://tracker.ceph.com/issues/37654
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>