ReplicatedPG: don't skip missing if sentries is empty on pgls

Formerly, if sentries is empty, we skip missing.  In general,
we need to continue adding items from missing until we get
to next (returned from collection_list_partial) to avoid
missing any objects.

Fixes: #6633
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>