[SPARK-4420][SQL] Change nullability of Cast from DoubleType/FloatType to DecimalType.

This is follow-up of [SPARK-4390](https://issues.apache.org/jira/browse/SPARK-4390) (#3256).

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #3278 from ueshin/issues/SPARK-4420 and squashes the following commits:

7fea558 [Takuya UESHIN] Add some tests.
cb2301a [Takuya UESHIN] Fix tests.
133bad5 [Takuya UESHIN] Change nullability of Cast from DoubleType/FloatType to DecimalType.

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

@@ -37,6 +37,8 @@ case class Cast(child: Expression, dataType: DataType) extends UnaryExpression w
     case (BooleanType, DateType)      => true
     case (DateType, _: NumericType)   => true
     case (DateType, BooleanType)      => true
+    case (DoubleType, _: DecimalType) => true
+    case (FloatType, _: DecimalType)  => true
     case (_, DecimalType.Fixed(_, _)) => true  // TODO: not all upcasts here can really give null
     case _                            => child.nullable
   }

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

@@ -347,8 +347,8 @@ class ExpressionEvaluationSuite extends FunSuite {
     // - Because of this, casts to fixed-precision decimals should be nullable
 
     assert(Cast(Literal(123), DecimalType.Unlimited).nullable === false)
-    assert(Cast(Literal(10.03f), DecimalType.Unlimited).nullable === false)
-    assert(Cast(Literal(10.03), DecimalType.Unlimited).nullable === false)
+    assert(Cast(Literal(10.03f), DecimalType.Unlimited).nullable === true)
+    assert(Cast(Literal(10.03), DecimalType.Unlimited).nullable === true)
     assert(Cast(Literal(Decimal(10.03)), DecimalType.Unlimited).nullable === false)
 
     assert(Cast(Literal(123), DecimalType(2, 1)).nullable === true)
@@ -396,6 +396,16 @@ class ExpressionEvaluationSuite extends FunSuite {
     checkEvaluation(Cast(Literal(-9.95), DecimalType(1, 0)), null)
     checkEvaluation(Cast(Literal(Decimal(-9.95)), DecimalType(3, 1)), Decimal(-10.0))
     checkEvaluation(Cast(Literal(Decimal(-9.95)), DecimalType(1, 0)), null)
+
+    checkEvaluation(Cast(Literal(Double.NaN), DecimalType.Unlimited), null)
+    checkEvaluation(Cast(Literal(1.0 / 0.0), DecimalType.Unlimited), null)
+    checkEvaluation(Cast(Literal(Float.NaN), DecimalType.Unlimited), null)
+    checkEvaluation(Cast(Literal(1.0f / 0.0f), DecimalType.Unlimited), null)
+
+    checkEvaluation(Cast(Literal(Double.NaN), DecimalType(2, 1)), null)
+    checkEvaluation(Cast(Literal(1.0 / 0.0), DecimalType(2, 1)), null)
+    checkEvaluation(Cast(Literal(Float.NaN), DecimalType(2, 1)), null)
+    checkEvaluation(Cast(Literal(1.0f / 0.0f), DecimalType(2, 1)), null)
   }
 
   test("timestamp") {