[SPARK-2509][SQL] Add optimization for Substring.

`Substring` including `null` literal cases could be added to `NullPropagation`.

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

Closes #1428 from ueshin/issues/SPARK-2509 and squashes the following commits:

d9eb85f [Takuya UESHIN] Add Substring cases to NullPropagation.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -171,6 +171,9 @@ object NullPropagation extends Rule[LogicalPlan] {
           case Literal(candidate, _) if candidate == v => true
           case _ => false
         })) => Literal(true, BooleanType)
+      case e @ Substring(Literal(null, _), _, _) => Literal(null, e.dataType)
+      case e @ Substring(_, Literal(null, _), _) => Literal(null, e.dataType)
+      case e @ Substring(_, _, Literal(null, _)) => Literal(null, e.dataType)
       // Put exceptional cases above if any
       case e: BinaryArithmetic => e.children match {
         case Literal(null, _) :: right :: Nil => Literal(null, e.dataType)