CometIcebergNativeSuite single-split sort-merge join DPP test fails on Spark 3.5 with AQE #6901

Describe the bug

DPP - single-split sort-merge join (AQE=true, pruned=a) and (AQE=true, pruned=b) in CometIcebergNativeSuite, added in #6814, fail on the Spark 3.5 profile:

List() was empty Expected AQE to replace the pruned join with an empty relation

Found in #5867: https://github.com/apache/datafusion-comet/actions/runs/38276011652/job/114910128309. #6814's own Spark 3.5 [scans] job failed on the same two tests (https://github.com/apache/datafusion-comet/actions/runs/38066634294/job/114798528903), and they fail the same way on main (38f394e). Spark 3.4, 4.0, 4.1 and 4.2 pass. The pull request tier runs only Spark 4.1, so main's CI doesn't run this profile.

The test asserts a CometEmptyRelationExec whenever isSpark35Plus && aqeEnabled && scans.isEmpty. On Spark 3.5, AQEPropagateEmptyRelation replaces the pruned join with an empty LocalRelation, which plans as LocalTableScanExec. The EmptyRelation node, and with it EmptyRelationExec, only arrived in Spark 4.0 (SPARK-48466), and ShimCometEmptyRelation returns None on 3.x. spark.comet.exec.localTableScan.enabled defaults to false, so the empty scan and the aggregates above it stay in Spark:

HashAggregate
+- ShuffleQueryStage
   +- Exchange SinglePartition
      +- HashAggregate
         +- LocalTableScan <empty>

The query result is correct. Only the plan check is wrong for 3.5.

Steps to reproduce

./mvnw test -Pspark-3.5 -Pscala-2.13 -Dtest=none \
  -Dsuites='org.apache.comet.CometIcebergNativeSuite DPP - single-split sort-merge join'

Expected behavior

The test passes on every Spark profile Comet builds.

Additional context

Requiring CometEmptyRelationExec only on Spark 4.0+ and accepting the empty LocalTableScanExec on 3.5 would match what each version plans.

Willingness to contribute

I can contribute a fix for this bug independently