[compare] Use const ref for iteration and check value before access

Speculative means of addressing a crash in the cluster manager. Rather
than copying pair objects, we'll just use references. We also check
that a product info has a value before trying to access it.

Bug: 401914721
Change-Id: Ic4b94be26365e427f20bbd6902d6cfde5e1887dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6339967
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Yue Zhang <yuezhanggg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1430365}