bug: null IN () returns false instead of null under legacy null-in-empty behavior #4786

Describe the bug

For a null operand against an empty IN list, Comet always returns false. Spark returns null when spark.sql.legacy.nullInEmptyListBehavior=true. Comet ignores the config and the null operand.

Steps to reproduce

Empty IN lists are not expressible in SQL, so build via the DataFrame API; ConvertToLocalRelation and OptimizeIn must be disabled or the IN is folded away. Add to a suite extending CometTestBase (config exists in Spark 4+):

test("null IN empty list under legacy behavior") {
  assume(org.apache.comet.CometSparkSessionExtensions.isSpark40Plus)
  withSQLConf(
    "spark.sql.optimizer.excludedRules" ->
      ("org.apache.spark.sql.catalyst.optimizer.ConvertToLocalRelation," +
        "org.apache.spark.sql.catalyst.optimizer.OptimizeIn"),
    "spark.sql.legacy.nullInEmptyListBehavior" -> "true") {
    val data: Seq[(Integer, Integer)] =
      Seq((Integer.valueOf(1), Integer.valueOf(1)), (null, Integer.valueOf(2)))
    withParquetTable(data, "t") {
      val df = sql("SELECT _1 AS a FROM t").select(col("a"), col("a").isin())
      checkSparkAnswer(df)
    }
  }
}

== Results ==
!== Spark Answer - 2 ==            == Comet Answer - 2 ==
 struct<a:int,(a IN ()):boolean>   struct<a:int,(a IN ()):boolean>
 [1,false]                         [1,false]
![null,null]                       [null,false]

Expected behavior

null IN () returns null under legacy behavior, matching Spark. (Under ANSI-on, the Spark 4 default, false is expected and Comet already matches, so only the legacy path is affected.)

Additional context

Found while enabling CometLocalTableScanExec by default (#4393), but reproduces over a Parquet scan. Upstream test: EmptyInSuite "IN with empty list".