Outer Join Improvements: Create bucket(s) for non-matching values

We need to create bucket(s) for non-matching values in single repartition outer joins.

The issue names single repartition outer joins but provides no files, entry points, or tests. Start by locating the relevant Citus outer-join implementation and existing tests, then establish the expected behavior for buckets containing non-matching values. Done means the behavior is implemented and covered by tests, but the issue does not define the exact expected cases.

Written by the indexing model from the issue text.