Fix joins to include partial null results

[jbrooks-stripe]

Summary

We found some online/offline inconsistencies with GroupBys when using multiple keys where some contained nulls. The online behavior would aggregate on (key1, null), whereas the offline would result in nulls if any keys were null.

This corrects the offline joins to match the online behavior of partial aggregations.

Why / Goal

Test Plan

• [x] Added Unit Tests
• [x] Covered by existing CI
• [x] Integration tested

Checklist

• [x] Documentation update

Reviewers