ReplicatedPG::finish_ctx: clear object_info if !obs.exists

Otherwise, we see a different object_info_t depending on whether the
transaction deleting the object clears before another op recreating it appears.
In particular, we use oi.version to set the prior_version on the log entries in
finish_ctx.  If the oi is allowed to stick around the recreation log event will
have a prior version of the deletion event when it should have a prior version
of eversion_t().

Fixes: #7655
Signed-off-by: Samuel Just <sam.just@inktank.com>