[VL] Unstable Spark UT

### Backend

VL (Velox)

### Bug description

https://github.com/apache/gluten/actions/runs/24952706498/job/73065994432?pr=11988
```
2026-04-26T10:23:14.1390962Z - SPARK-16644: Aggregate should not put aggregate expressions to constraints *** FAILED ***
2026-04-26T10:23:14.1392962Z java.lang.NullPointerException: Cannot invoke "org.apache.spark.sql.execution.SparkPlan.subqueries()" because "x$7" is null
2026-04-26T10:23:14.1395298Z at org.apache.spark.sql.execution.adaptive.AdaptiveSparkPlanHelper.$anonfun$subqueriesAll$1(AdaptiveSparkPlanHelper.scala:121)
2026-04-26T10:23:14.1397684Z at org.apache.spark.sql.execution.adaptive.AdaptiveSparkPlanHelper.$anonfun$flatMap$1(AdaptiveSparkPlanHelper.scala:75)
2026-04-26T10:23:14.1399606Z at org.apache.spark.sql.execution.adaptive.AdaptiveSparkPlanHelper.$anonfun$flatMap$1$adapted(AdaptiveSparkPlanHelper.scala:75)
2026-04-26T10:23:14.1401383Z at org.apache.spark.sql.execution.adaptive.AdaptiveSparkPlanHelper.foreach(AdaptiveSparkPlanHelper.scala:45)
2026-04-26T10:23:14.1402853Z at org.apache.spark.sql.execution.adaptive.AdaptiveSparkPlanHelper.foreach$(AdaptiveSparkPlanHelper.scala:44)
2026-04-26T10:23:14.1404026Z at org.apache.gluten.test.FallbackUtil$.foreach(FallbackUtil.scala:31)
2026-04-26T10:23:14.1405212Z at org.apache.spark.sql.execution.adaptive.AdaptiveSparkPlanHelper.flatMap(AdaptiveSparkPlanHelper.scala:75)
2026-04-26T10:23:14.1406736Z at org.apache.spark.sql.execution.adaptive.AdaptiveSparkPlanHelper.flatMap$(AdaptiveSparkPlanHelper.scala:73)
2026-04-26T10:23:14.1407966Z at org.apache.gluten.test.FallbackUtil$.flatMap(FallbackUtil.scala:31)
2026-04-26T10:23:14.1409251Z at org.apache.spark.sql.execution.adaptive.AdaptiveSparkPlanHelper.subqueriesAll(AdaptiveSparkPlanHelper.scala:121)
2026-04-26T10:23:14.1410234Z ...
```

### Gluten version

_No response_

### Spark version

None

### Spark configurations

_No response_

### System information

_No response_

### Relevant logs

```bash

```

Start with the linked GitHub Actions job and the SPARK-16644 failure, then inspect FallbackUtil.scala around the call into AdaptiveSparkPlanHelper.subqueriesAll at AdaptiveSparkPlanHelper.scala:121. Reproduce the unstable Spark unit test in the VL backend and trace why a null SparkPlan reaches subqueriesAll. Done means the test no longer fails intermittently and the relevant test run passes.

Written by the indexing model from the issue text.