Don't log old PartitionInfo for full payload.

For full payload we were logging something like:
PartitionInfo old system sha256:  size: 0
Which has no useful information and could lead to confusion.

Bug: 71913973
Test: brillo_update_payload verify with a full payload
Change-Id: I3ebf2e663907350387cfa0b93fdf2512423d39e3