JVM columnar shuffle fails with ArrayIndexOutOfBoundsException when spark.shuffle.checksum.enabled=false #6256

Describe the bug

With spark.shuffle.checksum.enabled=false, every task of a Comet JVM columnar shuffle that uses the sort-based writer fails:

java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0
	at org.apache.spark.shuffle.sort.SpillSorter.writeSortedFileNative(SpillSorter.java:290)
	at org.apache.spark.shuffle.sort.CometShuffleExternalSorter.closeAndGetSpills(CometShuffleExternalSorter.java:337)
	at org.apache.spark.sql.comet.execution.shuffle.CometUnsafeShuffleWriter.closeAndWriteOutput(CometUnsafeShuffleWriter.java:309)
	at org.apache.spark.sql.comet.execution.shuffle.CometUnsafeShuffleWriter.write(CometUnsafeShuffleWriter.java:255)

With checksums disabled, createPartitionChecksums returns an empty array (CometShuffleChecksumSupport.java#L28-L41). writeSortedFileNative guards every other access with partitionChecksums.length > 0, but not the store at SpillSorter.java#L290. That store runs every time the sorted records move to the next partition.

Steps to reproduce

With spark.shuffle.checksum.enabled=false in the SparkConf:

spark.conf.set("spark.comet.shuffle.mode", "jvm")
spark.range(0, 100000, 1, 4)
  .selectExpr("id", "cast(id as string) as s")
  .repartition(300, col("id"))
  .count()

300 partitions is above spark.shuffle.sort.bypassMergeThreshold, so Comet uses the sort-based writer.

Expected behavior

The shuffle succeeds with checksums disabled.

Additional context

The unguarded store has been there since at least #3081, which moved SpillSorter into its own class, so 1.0.0 and 1.1.0 are affected. SpillSorterSuite always passes a non-empty checksum array, which is why no test caught this.