## 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.
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