## What changes were proposed in this pull request?
This PR is proposing a fix for the output data type of ```If``` and ```CaseWhen``` expression. Upon till now, the implementation of exprassions has ignored nullability of nested types from different execution branches and returned the type of the first branch.
This could lead to an unwanted ```NullPointerException``` from other expressions depending on a ```If```/```CaseWhen``` expression.
Example:
```
val rows = new util.ArrayList[Row]()
rows.add(Row(true, ("a", 1)))
rows.add(Row(false, (null, 2)))
val schema = StructType(Seq(
StructField("cond", BooleanType, false),
StructField("s", StructType(Seq(
StructField("val1", StringType, true),
StructField("val2", IntegerType, false)
)), false)
))
val df = spark.createDataFrame(rows, schema)
df
.select(when('cond, struct(lit("x").as("val1"), lit(10).as("val2"))).otherwise('s) as "res")
.select('res.getField("val1"))
.show()
```
Exception:
```
Exception in thread "main" java.lang.NullPointerException
at org.apache.spark.sql.catalyst.expressions.codegen.UnsafeWriter.write(UnsafeWriter.java:109)
at org.apache.spark.sql.catalyst.expressions.GeneratedClass$SpecificUnsafeProjection.apply(Unknown Source)
at org.apache.spark.sql.execution.LocalTableScanExec$$anonfun$unsafeRows$1.apply(LocalTableScanExec.scala:44)
at org.apache.spark.sql.execution.LocalTableScanExec$$anonfun$unsafeRows$1.apply(LocalTableScanExec.scala:44)
...
```
Output schema:
```
root
|-- res.val1: string (nullable = false)
```
## How was this patch tested?
New test cases added into
- DataFrameSuite.scala
- conditionalExpressions.scala
Author: Marek Novotny <mn.mikke@gmail.com>
Closes#21687 from mn-mikke/SPARK-24165.
74a8d6308b