PGLog: allow for empty pg log in update_range if log_tail == eversion_t()

Signed-off-by: Samuel Just <sam.just@inktank.com>
(cherry picked from commit 03c5344f74991ec351cdc8a55f6495d496479091)

Fixes: 10168
Merge pull request #2982 from athanatos/wip-10168

PGLog: allow for empty pg log in update_range if log_tail == eversion_t(...

Reviewed-by: Sage Weil <sage@redhat.com>