[SPARK-27671][SQL] Fix error when casting from a nested null in a struct

When a null in a nested field in struct, casting from the struct throws error, currently.

```scala
scala> sql("select cast(struct(1, null) as struct<a:int,b:int>)").show
scala.MatchError: NullType (of class org.apache.spark.sql.types.NullType$)
  at org.apache.spark.sql.catalyst.expressions.Cast.castToInt(Cast.scala:447)
  at org.apache.spark.sql.catalyst.expressions.Cast.cast(Cast.scala:635)
  at org.apache.spark.sql.catalyst.expressions.Cast.$anonfun$castStruct$1(Cast.scala:603)
```

Similarly, inline table, which casts null in nested field under the hood, also throws an error.

```scala
scala> sql("select * FROM VALUES (('a', (10, null))), (('b', (10, 50))), (('c', null)) AS tab(x, y)").show
org.apache.spark.sql.AnalysisException: failed to evaluate expression named_struct('col1', 10, 'col2', NULL): NullType (of class org.apache.spark.sql.t
ypes.NullType$); line 1 pos 14
  at org.apache.spark.sql.catalyst.analysis.package$AnalysisErrorAt.failAnalysis(package.scala:47)
  at org.apache.spark.sql.catalyst.analysis.ResolveInlineTables.$anonfun$convert$6(ResolveInlineTables.scala:106)
```

This fixes the issue.

Added tests.

Closes apache#24576 from viirya/cast-null.

Authored-by: Liang-Chi Hsieh <viirya@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
(cherry picked from commit 8b0bdaa)
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

Author: viirya

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

@@ -607,6 +607,12 @@ case class Cast(child: Expression, dataType: DataType, timeZoneId: Option[String
     // We can return what the children return. Same thing should happen in the codegen path.
     if (DataType.equalsStructurally(from, to)) {
       identity
+    } else if (from == NullType) {
+      // According to `canCast`, NullType can be casted to any type.
+      // For primitive types, we don't reach here because the guard of `nullSafeEval`.
+      // But for nested types like struct, we might reach here for nested null type field.
+      // We won't call the returned function actually, but returns a placeholder.
+      _ => throw new SparkException(s"should not directly cast from NullType to $to.")
     } else {
       to match {
         case dt if dt == from => identity[Any]

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CastSuite.scala

@@ -989,4 +989,19 @@ class CastSuite extends SparkFunSuite with ExpressionEvalHelper {
       }
     }
   }
+
+  test("SPARK-27671: cast from nested null type in struct") {
+    import DataTypeTestUtils._
+
+    atomicTypes.foreach { atomicType =>
+      val struct = Literal.create(
+        InternalRow(null),
+        StructType(Seq(StructField("a", NullType, nullable = true))))
+
+      val ret = cast(struct, StructType(Seq(
+        StructField("a", atomicType, nullable = true))))
+      assert(ret.resolved)
+      checkEvaluation(ret, InternalRow(null))
+    }
+  }
 }

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

@@ -2622,4 +2622,13 @@ class DataFrameSuite extends QueryTest with SharedSQLContext {
       checkAnswer(res, Row("1-1", 6, 6))
     }
   }
+
+  test("SPARK-27671: Fix analysis exception when casting null in nested field in struct") {
+    val df = sql("SELECT * FROM VALUES (('a', (10, null))), (('b', (10, 50))), " +
+      "(('c', null)) AS tab(x, y)")
+    checkAnswer(df, Row("a", Row(10, null)) :: Row("b", Row(10, 50)) :: Row("c", null) :: Nil)
+
+    val cast = sql("SELECT cast(struct(1, null) AS struct<a:int,b:int>)")
+    checkAnswer(cast, Row(Row(1, null)) :: Nil)
+  }
 }