## What changes were proposed in this pull request?

Since `stack` function generates a table with nullable columns, it should allow mixed null values.

```scala
scala> sql("select stack(3, 1, 2, 3)").printSchema
root
 |-- col0: integer (nullable = true)

scala> sql("select stack(3, 1, 2, null)").printSchema
org.apache.spark.sql.AnalysisException: cannot resolve 'stack(3, 1, 2, NULL)' due to data type mismatch: Argument 1 (IntegerType) != Argument 3 (NullType); line 1 pos 7;
```

## How was this patch tested?

Pass the Jenkins with a new test case.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #17251 from dongjoon-hyun/SPARK-19910.