[SPARK-39496][SQL][3.1] Handle null struct in Inline.eval

[bersprockets]

What changes were proposed in this pull request?

Backport of #36903

Change Inline.eval to return a row of null values rather than a null row in the case of a null input struct.

Why are the changes needed?

Consider the following query:

set spark.sql.codegen.wholeStage=false;
select inline(array(named_struct('a', 1, 'b', 2), null));

This query fails with a NullPointerException:

22/06/16 15:10:06 ERROR Executor: Exception in task 0.0 in stage 0.0 (TID 0)
java.lang.NullPointerException
	at org.apache.spark.sql.catalyst.expressions.GeneratedClass$SpecificUnsafeProjection.apply(Unknown Source)
	at org.apache.spark.sql.execution.GenerateExec.$anonfun$doExecute$11(GenerateExec.scala:122)

(In Spark 3.1.x, you don't need to set spark.sql.codegen.wholeStage to false to reproduce the error, since Spark 3.1.x has no codegen path for Inline).

This query fails regardless of the setting of spark.sql.codegen.wholeStage:

val dfWide = (Seq((1))
  .toDF("col0")
  .selectExpr(Seq.tabulate(99)(x => s"$x as col${x + 1}"): _*))

val df = (dfWide
  .selectExpr("*", "array(named_struct('a', 1, 'b', 2), null) as struct_array"))

df.selectExpr("*", "inline(struct_array)").collect

It fails with

22/06/16 15:18:55 ERROR Executor: Exception in task 0.0 in stage 0.0 (TID 0)/ 1]
java.lang.NullPointerException
	at org.apache.spark.sql.catalyst.expressions.JoinedRow.isNullAt(JoinedRow.scala:80)
	at org.apache.spark.sql.catalyst.expressions.GeneratedClass$SpecificUnsafeProjection.writeFields_0_8$(Unknown Source)

When Inline.eval returns a null row in the collection, GenerateExec gets a NullPointerException either when joining the null row with required child output, or projecting the null row.

This PR avoids producing the null row and produces a row of null values instead:

spark-sql> set spark.sql.codegen.wholeStage=false;
spark.sql.codegen.wholeStage	false
Time taken: 3.095 seconds, Fetched 1 row(s)
spark-sql> select inline(array(named_struct('a', 1, 'b', 2), null));
1	2
NULL	NULL
Time taken: 1.214 seconds, Fetched 2 row(s)
spark-sql>

Does this PR introduce any user-facing change?

No.

How was this patch tested?

New unit test.

[HyukjinKwon]

Merged to branch-3.1.