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

### What changes were proposed in this pull request?

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 3.1.3, you don't need to set `spark.sql.codegen.wholeStage` to false to reproduce the error, since Spark 3.1.3 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.

Closes #36903 from bersprockets/inline_eval_null_struct_issue.

Authored-by: Bruce Robbins <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>
(cherry picked from commit c4d5390)
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: bersprockets

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/generators.scala

@@ -452,13 +452,17 @@ case class Inline(child: Expression) extends UnaryExpression with CollectionGene
 
   private lazy val numFields = elementSchema.fields.length
 
+  private lazy val generatorNullRow = new GenericInternalRow(elementSchema.length)
+
   override def eval(input: InternalRow): TraversableOnce[InternalRow] = {
     val inputArray = child.eval(input).asInstanceOf[ArrayData]
     if (inputArray == null) {
       Nil
     } else {
-      for (i <- 0 until inputArray.numElements())
-        yield inputArray.getStruct(i, numFields)
+      for (i <- 0 until inputArray.numElements()) yield {
+        val s = inputArray.getStruct(i, numFields)
+        if (s == null) generatorNullRow else s
+      }
     }
   }
 

sql/core/src/test/scala/org/apache/spark/sql/GeneratorFunctionSuite.scala

@@ -23,6 +23,7 @@ import org.apache.spark.sql.catalyst.expressions.codegen.{CodegenContext, ExprCo
 import org.apache.spark.sql.catalyst.expressions.codegen.Block._
 import org.apache.spark.sql.catalyst.trees.LeafLike
 import org.apache.spark.sql.functions._
+import org.apache.spark.sql.internal.SQLConf
 import org.apache.spark.sql.test.SharedSparkSession
 import org.apache.spark.sql.types.{IntegerType, StructType}
 
@@ -389,7 +390,7 @@ class GeneratorFunctionSuite extends QueryTest with SharedSparkSession {
     }
   }
 
-  test("SPARK-39061: inline should handle null struct") {
+  def testNullStruct(): Unit = {
     val df = sql(
       """select * from values
         |(
@@ -413,6 +414,16 @@ class GeneratorFunctionSuite extends QueryTest with SharedSparkSession {
       sql("select a, inline(b) from t1"),
       Row(1, 0, 1) :: Row(1, null, null) :: Row(1, 2, 3) :: Row(1, null, null) :: Nil)
   }
+
+  test("SPARK-39061: inline should handle null struct") {
+    testNullStruct
+  }
+
+  test("SPARK-39496: inline eval path should handle null struct") {
+    withSQLConf(SQLConf.WHOLESTAGE_CODEGEN_ENABLED.key -> "false") {
+      testNullStruct
+    }
+  }
 }
 
 case class EmptyGenerator() extends Generator with LeafLike[Expression] {