[SQL][MINOR] Defensive code for number of Partitions in rCTEs

[Pajaraja]

What changes were proposed in this pull request?

Set the number of partitions in recursive CTEs to be at least 1.

Why are the changes needed?

Defensive programming to avoid zero-partition RDDs, which could cause potential issues in downstream operations like coalesce.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Existing CIs.

Was this patch authored or co-authored using generative AI tooling?

No.

[cloud-fan]

Is it possible to add a unit test by building physical plans directly? e.g. RDDScanExec can host a 0-partition RDD.