[VL] Flaky string sentences function test

### Backend

VL (Velox)

### Bug description

```
2025-07-15T18:23:19.4895514Z - string sentences function *** FAILED ***
2025-07-15T18:23:19.4898068Z org.apache.spark.SparkException: Job aborted due to stage failure: Task 0 in stage 230.0 failed 1 times, most recent failure: Lost task 0.0 in stage 230.0 (TID 292) (905469d1b180 executor driver): java.lang.NullPointerException: Cannot invoke "org.apache.spark.sql.catalyst.util.ArrayData.numElements()" because "" is null
2025-07-15T18:23:19.4900984Z at org.apache.spark.sql.catalyst.expressions.GeneratedClass$SpecificSafeProjection.MapObjects_3$(Unknown Source)
2025-07-15T18:23:19.4902251Z at org.apache.spark.sql.catalyst.expressions.GeneratedClass$SpecificSafeProjection.apply(Unknown Source)
2025-07-15T18:23:19.4902954Z at scala.collection.Iterator$$anon$10.next(Iterator.scala:461)
2025-07-15T18:23:19.4903400Z at scala.collection.Iterator$$anon$10.next(Iterator.scala:461)
2025-07-15T18:23:19.4903910Z at org.apache.spark.util.Iterators$.size(Iterators.scala:31)
2025-07-15T18:23:19.4904519Z at org.apache.spark.util.Utils$.getIteratorSize(Utils.scala:1787)
2025-07-15T18:23:19.4905670Z at org.apache.spark.rdd.RDD.$anonfun$count$1(RDD.scala:1296)
2025-07-15T18:23:19.4906323Z at org.apache.spark.rdd.RDD.$anonfun$count$1$adapted(RDD.scala:1296)
2025-07-15T18:23:19.4907105Z at org.apache.spark.SparkContext.$anonfun$runJob$5(SparkContext.scala:2433)
2025-07-15T18:23:19.4907859Z at org.apache.spark.scheduler.ResultTask.runTask(ResultTask.scala:93)
2025-07-15T18:23:19.4908666Z at org.apache.spark.TaskContext.runTaskWithListeners(TaskContext.scala:166)
2025-07-15T18:23:19.4909664Z at org.apache.spark.scheduler.Task.run(Task.scala:141)
2025-07-15T18:23:19.4910423Z at org.apache.spark.executor.Executor$TaskRunner.$anonfun$run$4(Executor.scala:620)
2025-07-15T18:23:19.4911372Z at org.apache.spark.util.SparkErrorUtils.tryWithSafeFinally(SparkErrorUtils.scala:64)
2025-07-15T18:23:19.4912401Z at org.apache.spark.util.SparkErrorUtils.tryWithSafeFinally$(SparkErrorUtils.scala:61)
2025-07-15T18:23:19.4913257Z at org.apache.spark.util.Utils$.tryWithSafeFinally(Utils.scala:94)
2025-07-15T18:23:19.4913952Z at org.apache.spark.executor.Executor$TaskRunner.run(Executor.scala:623)
2025-07-15T18:23:19.4915094Z at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
2025-07-15T18:23:19.4916103Z at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
2025-07-15T18:23:19.4916863Z at java.base/java.lang.Thread.run(Thread.java:833)
2025-07-15T18:23:19.4917193Z
2025-07-15T18:23:19.4917306Z Driver stacktrace:
2025-07-15T18:23:19.4918051Z at org.apache.spark.scheduler.DAGScheduler.failJobAndIndependentStages(DAGScheduler.scala:2856)
2025-07-15T18:23:19.4919172Z at org.apache.spark.scheduler.DAGScheduler.$anonfun$abortStage$2(DAGScheduler.scala:2792)
2025-07-15T18:23:19.4920417Z at org.apache.spark.scheduler.DAGScheduler.$anonfun$abortStage$2$adapted(DAGScheduler.scala:2791)
2025-07-15T18:23:19.4921436Z at scala.collection.mutable.ResizableArray.foreach(ResizableArray.scala:62)
2025-07-15T18:23:19.4922357Z at scala.collection.mutable.ResizableArray.foreach$(ResizableArray.scala:55)
2025-07-15T18:23:19.4923384Z at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:49)
2025-07-15T18:23:19.4924280Z at org.apache.spark.scheduler.DAGScheduler.abortStage(DAGScheduler.scala:2791)
2025-07-15T18:23:19.4925321Z at org.apache.spark.scheduler.DAGScheduler.$anonfun$handleTaskSetFailed$1(DAGScheduler.scala:1247)
2025-07-15T18:23:19.4926523Z at org.apache.spark.scheduler.DAGScheduler.$anonfun$handleTaskSetFailed$1$adapted(DAGScheduler.scala:1247)
2025-07-15T18:23:19.4927354Z at scala.Option.foreach(Option.scala:407)
2025-07-15T18:23:19.4927780Z ...
2025-07-15T18:23:19.4928128Z Cause: java.lang.NullPointerException:
2025-07-15T18:23:19.4929083Z at org.apache.spark.sql.catalyst.expressions.GeneratedClass$SpecificSafeProjection.MapObjects_3$(Unknown Source)
2025-07-15T18:23:19.4930839Z at org.apache.spark.sql.catalyst.expressions.GeneratedClass$SpecificSafeProjection.apply(Unknown Source)
2025-07-15T18:23:19.4932093Z at scala.collection.Iterator$$anon$10.next(Iterator.scala:461)
2025-07-15T18:23:19.4933088Z at scala.collection.Iterator$$anon$10.next(Iterator.scala:461)
2025-07-15T18:23:19.4933915Z at org.apache.spark.util.Iterators$.size(Iterators.scala:31)
2025-07-15T18:23:19.4934737Z at org.apache.spark.util.Utils$.getIteratorSize(Utils.scala:1787)
2025-07-15T18:23:19.4935535Z at org.apache.spark.rdd.RDD.$anonfun$count$1(RDD.scala:1296)
2025-07-15T18:23:19.4936347Z at org.apache.spark.rdd.RDD.$anonfun$count$1$adapted(RDD.scala:1296)
2025-07-15T18:23:19.4937492Z at org.apache.spark.SparkContext.$anonfun$runJob$5(SparkContext.scala:2433)
2025-07-15T18:23:19.4938252Z at org.apache.spark.scheduler.ResultTask.runTask(ResultTask.scala:93)
```

### Gluten version

_No response_

### Spark version

None

### Spark configurations

_No response_

### System information

_No response_

### Relevant logs

```bash

```