[SPARK-3362][SQL] Fix resolution for casewhen with nulls.

Current implementation will ignore else val type.

Author: Daoyuan Wang <daoyuan.wang@intel.com>

Closes #2245 from adrian-wang/casewhenbug and squashes the following commits:

3332f6e [Daoyuan Wang] remove wrong comment
83b536c [Daoyuan Wang] a comment to trigger retest
d7315b3 [Daoyuan Wang] code improve
eed35fc [Daoyuan Wang] bug in casewhen resolve

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

@@ -265,12 +265,13 @@ case class CaseWhen(branches: Seq[Expression]) extends Expression {
       false
     } else {
       val allCondBooleans = predicates.forall(_.dataType == BooleanType)
-      val dataTypesEqual = values.map(_.dataType).distinct.size <= 1
+      // both then and else val should be considered.
+      val dataTypesEqual = (values ++ elseValue).map(_.dataType).distinct.size <= 1
       allCondBooleans && dataTypesEqual
     }
   }
 
-  /** Written in imperative fashion for performance considerations.  Same for CaseKeyWhen. */
+  /** Written in imperative fashion for performance considerations. */
   override def eval(input: Row): Any = {
     val len = branchesArr.length
     var i = 0

sql/hive/src/test/resources/golden/case when then 1 else null end -0-f7c7fdd35c084bc797890aa08d33693c

+1

sql/hive/src/test/resources/golden/case when then 1.0 else null end -0-aeb1f906bfe92f2d406f84109301afe0

+1.0

sql/hive/src/test/resources/golden/case when then 1L else null end -0-763ae85e7a52b4cf4162d6a8931716bb

+1

sql/hive/src/test/resources/golden/case when then 1S else null end -0-6f5f3b3dbe9f1d1eb98443aef315b982

+1

sql/hive/src/test/resources/golden/case when then 1Y else null end -0-589982a400d86157791c7216b10b6b5d

+1

sql/hive/src/test/resources/golden/case when then null else 1 end -0-48bd83660cf3ba93cdbdc24559092171

+NULL

sql/hive/src/test/resources/golden/case when then null else 1.0 end -0-7f5ce763801781cf568c6a31dd80b623

+NULL

sql/hive/src/test/resources/golden/case when then null else 1L end -0-a7f1305ea4f86e596c368e35e45cc4e5

+NULL

sql/hive/src/test/resources/golden/case when then null else 1S end -0-dfb61969e6cb6e6dbe89225b538c8d98

+NULL

sql/hive/src/test/resources/golden/case when then null else 1Y end -0-7f4c32299c3738739b678ece62752a7b

+NULL

sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/HiveTypeCoercionSuite.scala

@@ -33,6 +33,12 @@ class HiveTypeCoercionSuite extends HiveComparisonTest {
     }
   }
 
+  val nullVal = "null"
+  baseTypes.init.foreach { i =>
+    createQueryTest(s"case when then $i else $nullVal end ", s"SELECT case when true then $i else $nullVal end FROM src limit 1")
+    createQueryTest(s"case when then $nullVal else $i end ", s"SELECT case when true then $nullVal else $i end FROM src limit 1")
+  }
+
   test("[SPARK-2210] boolean cast on boolean value should be removed") {
     val q = "select cast(cast(key=0 as boolean) as boolean) from src"
     val project = TestHive.sql(q).queryExecution.executedPlan.collect { case e: Project => e }.head