storage/pg: ignore main source output in resume upper calculation

[petrosagg]

The "main source output" never contains any data and therefore its upper is irrelevant when determining at which point to resume reading the replication stream. Separately, the assertion that it always exists in the configuration isn't always true, leading to flaky CI tests.

Fixes #9302

Motivation

Tips for reviewer

Checklist

• [ ] This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• [ ] This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• [ ] If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• [ ] If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• [ ] If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.