fix(substrait): roundtrip scalar subqueries in projections

[bvolpato]

Which issue does this PR close?

• Closes [substrait] Scalar subquery in select not supported #18066.

Rationale for this change

With datafusion.optimizer.enable_physical_uncorrelated_scalar_subquery disabled, DataFusion rewrites a scalar subquery in a projection as a left join. The optimizer can remove its true filter before Substrait serialization. This left JoinRel.expression unset, even though Substrait requires it, and the consumer failed with Plan("join condition should not be empty").

#18068 tried to fix the same roundtrip in the consumer, but also treated a conditionless inner JoinRel as a cross join. Review questioned that behavior change, then the PR went stale. This version does not reinterpret an invalid JoinRel: the producer uses CrossRel for conditionless inner joins and a literal true expression for conditionless non-inner joins.

What changes are included in this PR?

• Serialize conditionless inner joins as CrossRel.
• Serialize a literal true expression for conditionless non-inner joins.
• Add a scalar-subquery projection roundtrip test that checks the consumed plan and query result.

Are these changes tested?

• cargo test -p datafusion-substrait --features protoc
• cargo clippy --all-targets --all-features -- -D warnings

Are there any user-facing changes?

Scalar subqueries in projections now roundtrip when this optimizer option is disabled. No public API change.