[SPARK-2968][SQL] Fix nullabilities of Explode.

Output nullabilities of `Explode` could be detemined by `ArrayType.containsNull` or `MapType.valueContainsNull`.

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

Closes #1888 from ueshin/issues/SPARK-2968 and squashes the following commits:

d128c95 [Takuya UESHIN] Fix nullability of Explode.

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

@@ -86,19 +86,19 @@ case class Explode(attributeNames: Seq[String], child: Expression)
     (child.dataType.isInstanceOf[ArrayType] || child.dataType.isInstanceOf[MapType])
 
   private lazy val elementTypes = child.dataType match {
-    case ArrayType(et, _) => et :: Nil
-    case MapType(kt,vt, _) => kt :: vt :: Nil
+    case ArrayType(et, containsNull) => (et, containsNull) :: Nil
+    case MapType(kt, vt, valueContainsNull) => (kt, false) :: (vt, valueContainsNull) :: Nil
   }
 
   // TODO: Move this pattern into Generator.
   protected def makeOutput() =
     if (attributeNames.size == elementTypes.size) {
       attributeNames.zip(elementTypes).map {
-        case (n, t) => AttributeReference(n, t, nullable = true)()
+        case (n, (t, nullable)) => AttributeReference(n, t, nullable)()
       }
     } else {
       elementTypes.zipWithIndex.map {
-        case (t, i) => AttributeReference(s"c_$i", t, nullable = true)()
+        case ((t, nullable), i) => AttributeReference(s"c_$i", t, nullable)()
       }
     }