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
diff --git a/payload_consumer/delta_performer.cc b/payload_consumer/delta_performer.cc
index d5b0d58..0fb0a77 100644
--- a/payload_consumer/delta_performer.cc
+++ b/payload_consumer/delta_performer.cc
@@ -410,8 +410,10 @@
 
 void LogPartitionInfo(const vector<PartitionUpdate>& partitions) {
   for (const PartitionUpdate& partition : partitions) {
-    LogPartitionInfoHash(partition.old_partition_info(),
-                         "old " + partition.partition_name());
+    if (partition.has_old_partition_info()) {
+      LogPartitionInfoHash(partition.old_partition_info(),
+                           "old " + partition.partition_name());
+    }
     LogPartitionInfoHash(partition.new_partition_info(),
                          "new " + partition.partition_name());
   }