[SPARK-31229][SQL][TESTS] Add unit tests TypeCoercion.findTypeForComplex and Cast.canCast in null <> complex types

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

This PR (SPARK-31229) is rather a followup of #27926 (SPARK-31166). It adds unittests for `TypeCoercion.findTypeForComplex` and `Cast.canCast` about struct, map and array with the respect to null types.

### Why are the changes needed?

To detect which scope was broken in the future easily.

### Does this PR introduce any user-facing change?

No, it's a test-only.

### How was this patch tested?

Unittests were added.

Closes #27990 from HyukjinKwon/SPARK-31166-followup.

Authored-by: HyukjinKwon <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>
(cherry picked from commit bd32400)
Signed-off-by: HyukjinKwon <[email protected]>

Author: HyukjinKwon

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/analysis/TypeCoercionSuite.scala

@@ -467,18 +467,36 @@ class TypeCoercionSuite extends AnalysisTest {
       ArrayType(IntegerType, containsNull = false),
       Some(ArrayType(IntegerType, containsNull = true)))
 
+    widenTest(
+      ArrayType(NullType, containsNull = true),
+      ArrayType(IntegerType, containsNull = false),
+      Some(ArrayType(IntegerType, containsNull = true)))
+
     widenTest(
       MapType(IntegerType, StringType, valueContainsNull = true),
       MapType(IntegerType, StringType, valueContainsNull = false),
       Some(MapType(IntegerType, StringType, valueContainsNull = true)))
 
+    widenTest(
+      MapType(NullType, NullType, true),
+      MapType(IntegerType, StringType, false),
+      Some(MapType(IntegerType, StringType, true)))
+
     widenTest(
       new StructType()
         .add("arr", ArrayType(IntegerType, containsNull = true), nullable = false),
       new StructType()
         .add("arr", ArrayType(IntegerType, containsNull = false), nullable = true),
       Some(new StructType()
         .add("arr", ArrayType(IntegerType, containsNull = true), nullable = true)))
+
+    widenTest(
+      new StructType()
+        .add("null", NullType, nullable = true),
+      new StructType()
+        .add("null", IntegerType, nullable = false),
+      Some(new StructType()
+        .add("null", IntegerType, nullable = true)))
   }
 
   test("wider common type for decimal and array") {

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

@@ -1140,6 +1140,23 @@ class CastSuite extends CastSuiteBase {
     assert(Cast.canCast(set.dataType, ArrayType(StringType, false)))
   }
 
+  test("NullTypes should be able to cast to any complex types") {
+    assert(Cast.canCast(ArrayType(NullType, true), ArrayType(IntegerType, true)))
+    assert(Cast.canCast(ArrayType(NullType, false), ArrayType(IntegerType, true)))
+
+    assert(Cast.canCast(
+      MapType(NullType, NullType, true), MapType(IntegerType, IntegerType, true)))
+    assert(Cast.canCast(
+      MapType(NullType, NullType, false), MapType(IntegerType, IntegerType, true)))
+
+    assert(Cast.canCast(
+      StructType(StructField("a", NullType, true) :: Nil),
+      StructType(StructField("a", IntegerType, true) :: Nil)))
+    assert(Cast.canCast(
+      StructType(StructField("a", NullType, false) :: Nil),
+      StructType(StructField("a", IntegerType, true) :: Nil)))
+  }
+
   test("Cast should output null for invalid strings when ANSI is not enabled.") {
     withSQLConf(SQLConf.ANSI_ENABLED.key -> "false") {
       checkEvaluation(cast("abdef", DecimalType.USER_DEFAULT), null)