[SPARK-4828] [SQL] sum and avg on empty table should always return null

So the optimizations are not valid. Also I think the optimization here is rarely encounter, so removing them will not have influence on performance.

Can we merge #3445 before I add a comparison test case from this?

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

Closes #3675 from adrian-wang/sumempty and squashes the following commits:

42df763 [Daoyuan Wang] sum and avg on empty table should always return null

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

@@ -203,8 +203,6 @@ object NullPropagation extends Rule[LogicalPlan] {
   def apply(plan: LogicalPlan): LogicalPlan = plan transform {
     case q: LogicalPlan => q transformExpressionsUp {
       case e @ Count(Literal(null, _)) => Cast(Literal(0L), e.dataType)
-      case e @ Sum(Literal(c, _)) if c == 0 => Cast(Literal(0L), e.dataType)
-      case e @ Average(Literal(c, _)) if c == 0 => Literal(0.0, e.dataType)
       case e @ IsNull(c) if !c.nullable => Literal(false, BooleanType)
       case e @ IsNotNull(c) if !c.nullable => Literal(true, BooleanType)
       case e @ GetItem(Literal(null, _), _) => Literal(null, e.dataType)