[SPARK-40963][SQL] Set nullable correctly in project created by ExtractGenerator

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

When creating the project list for the new projection In `ExtractGenerator`, take into account whether the generator is outer when setting nullable on generator-related output attributes.

### Why are the changes needed?

This PR fixes an issue that can produce either incorrect results or a `NullPointerException`. It's a bit of an obscure issue in that I am hard-pressed to reproduce without using a subquery that has a inline table.

Example:
```
select c1, explode(c4) as c5 from (
  select c1, array(c3) as c4 from (
    select c1, explode_outer(c2) as c3
    from values
    (1, array(1, 2)),
    (2, array(2, 3)),
    (3, null)
    as data(c1, c2)
  )
);

+---+---+
|c1 |c5 |
+---+---+
|1  |1  |
|1  |2  |
|2  |2  |
|2  |3  |
|3  |0  |
+---+---+
```
In the last row, `c5` is 0, but should be `NULL`.

Another example:
```
select c1, exists(c4, x -> x is null) as c5 from (
  select c1, array(c3) as c4 from (
    select c1, explode_outer(c2) as c3
    from values
    (1, array(1, 2)),
    (2, array(2, 3)),
    (3, array())
    as data(c1, c2)
  )
);

+---+-----+
|c1 |c5   |
+---+-----+
|1  |false|
|1  |false|
|2  |false|
|2  |false|
|3  |false|
+---+-----+
```
In the last row, `false` should be `true`.

In both cases, at the time `CreateArray(c3)` is instantiated, `c3`'s nullability is incorrect because the new projection created by `ExtractGenerator` uses `generatorOutput` from `explode_outer(c2)` as a projection list. `generatorOutput` doesn't take into account that `explode_outer(c2)` is an _outer_ explode, so the nullability setting is lost.

`UpdateAttributeNullability` will eventually fix the nullable setting for attributes referring to `c3`, but it doesn't fix the `containsNull` setting for `c4` in `explode(c4)` (from the first example) or `exists(c4, x -> x is null)` (from the second example).

This example fails with a `NullPointerException`:
```
select c1, inline_outer(c4) from (
  select c1, array(c3) as c4 from (
    select c1, explode_outer(c2) as c3
    from values
    (1, array(named_struct('a', 1, 'b', 2))),
    (2, array(named_struct('a', 3, 'b', 4), named_struct('a', 5, 'b', 6))),
    (3, array())
    as data(c1, c2)
  )
);
22/10/30 17:34:42 ERROR Executor: Exception in task 1.0 in stage 8.0 (TID 14)
java.lang.NullPointerException
	at org.apache.spark.sql.catalyst.expressions.GeneratedClass$GeneratedIteratorForCodegenStage1.generate_doConsume_1$(Unknown Source)
	at org.apache.spark.sql.catalyst.expressions.GeneratedClass$GeneratedIteratorForCodegenStage1.generate_doConsume_0$(Unknown Source)
	at org.apache.spark.sql.catalyst.expressions.GeneratedClass$GeneratedIteratorForCodegenStage1.processNext(Unknown Source)
	at org.apache.spark.sql.execution.BufferedRowIterator.hasNext(BufferedRowIterator.java:43)
	at org.apache.spark.sql.execution.WholeStageCodegenExec$$anon$1.hasNext(WholeStageCodegenExec.scala:760)
	at org.apache.spark.sql.execution.SparkPlan.$anonfun$getByteArrayRdd$1(SparkPlan.scala:364)
```

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?

New unit test.

Closes apache#38440 from bersprockets/SPARK-40963.

Authored-by: Bruce Robbins <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: bersprockets

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/Analyzer.scala

@@ -2910,7 +2910,7 @@ class Analyzer(override val catalogManager: CatalogManager)
                   generatorOutput = ResolveGenerate.makeGeneratorOutput(generator, names),
                   child)
 
-                (Some(g), res._2 ++ g.generatorOutput)
+                (Some(g), res._2 ++ g.nullableOutput)
               case other =>
                 (res._1, res._2 :+ other)
             }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala

@@ -273,16 +273,17 @@ case class Generate(
 
   override def producedAttributes: AttributeSet = AttributeSet(generatorOutput)
 
+  def nullableOutput: Seq[Attribute] = {
+    generatorOutput.map { a =>
+      a.withNullability(outer || a.nullable)
+    }
+  }
+
   def qualifiedGeneratorOutput: Seq[Attribute] = {
-    val qualifiedOutput = qualifier.map { q =>
+    qualifier.map { q =>
       // prepend the new qualifier to the existed one
-      generatorOutput.map(a => a.withQualifier(Seq(q)))
-    }.getOrElse(generatorOutput)
-    val nullableOutput = qualifiedOutput.map {
-      // if outer, make all attributes nullable, otherwise keep existing nullability
-      a => a.withNullability(outer || a.nullable)
-    }
-    nullableOutput
+      nullableOutput.map(a => a.withQualifier(Seq(q)))
+    }.getOrElse(nullableOutput)
   }
 
   def output: Seq[Attribute] = requiredChildOutput ++ qualifiedGeneratorOutput

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

@@ -425,6 +425,25 @@ class GeneratorFunctionSuite extends QueryTest with SharedSparkSession {
       testNullStruct
     }
   }
+
+  test("SPARK-40963: generator output has correct nullability") {
+    // This test does not check nullability directly. Before SPARK-40963,
+    // the below query got wrong results due to incorrect nullability.
+    val df = sql(
+      """select c1, explode(c4) as c5 from (
+        |  select c1, array(c3) as c4 from (
+        |    select c1, explode_outer(c2) as c3
+        |    from values
+        |    (1, array(1, 2)),
+        |    (2, array(2, 3)),
+        |    (3, null)
+        |    as data(c1, c2)
+        |  )
+        |)
+        |""".stripMargin)
+    checkAnswer(df,
+      Row(1, 1) :: Row(1, 2) :: Row(2, 2) :: Row(2, 3) :: Row(3, null) :: Nil)
+  }
 }
 
 case class EmptyGenerator() extends Generator with LeafLike[Expression] {