rbd_mirror: avoid passing empty remote_mirror_uuid to group_replayer

rbd_mirror: avoid passing empty remote_mirror_uuid to group_replayer

group_replayer can fetch remote_mirror_uuid as remote_pool_meta.mirror_uuid

>>> gc = rbd.Group(ioctx, 'test_group')
>>> print(gc.group_snap_get_mirror_namespace('104b430672cf'))
{'state': 2, 'mirror_peer_uuids': [],
'primary_mirror_uuid': '6cd393ad-c21d-42e6-a404-0dabf596bfe7',
'primary_snap_id': '104b430672cf'}

Thanks to Ilya for highlighting the issue.

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>