rgw: apply_olh_log ignores RGW_ATTR_OLH_VER decode error

rgw: apply_olh_log ignores RGW_ATTR_OLH_VER decode error

this resolves test failures that return 400 Bad Request after:

  apply_olh_log failed to decode olh ver ''

because olh_init_modification_impl() is writing an empty attribute:

  bufferlist verbl;
  op.setxattr(RGW_ATTR_OLH_VER, verbl);

on such decode errors, link_epoch will be set to 0, and will correctly
be overwritten by real epochs from the olh log

Fixes: https://tracker.ceph.com/issues/39142
Signed-off-by: Casey Bodley <cbodley@redhat.com>