Nodereference sync needs to account for empty fields

When using cck content_multigroup and i18nsync, problems arise when creating translations because some nodereference fields can be null and need to remain null. But instead they are being disregarded.

For example, on my source node I have:

Content Multigroups:

noderef_field: (empty)
text_field: A

noderef_field: 1
text_field: B

noderef_field: (empty)
text_field: C

noderef_field: 2
text_field: D

When I create a synched translation I end up with:

Content Multigroups:

noderef_field: 1
text_field: A

noderef_field: 2
text_field: B

noderef_field: (empty)
text_field: C

noderef_field: (empty)
text_field: D

Because i18nsync_node_translation_nodereference_field() doesn't account for the empty node references, the deltas become incorrect and out of sync, putting the values into the wrong mulitgroup.