[SQL] Minor: Avoid calling Seq#size in a loop

Just found this instance while doing some jstack-based profiling of a Spark SQL job. It is very unlikely that this is causing much of a perf issue anywhere, but it is unnecessarily suboptimal.

Author: Aaron Davidson <aaron@databricks.com>

Closes #3593 from aarondav/seq-opt and squashes the following commits:

962cdfc [Aaron Davidson] [SQL] Minor: Avoid calling Seq#size in a loop

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

@@ -45,9 +45,9 @@ case class Coalesce(children: Seq[Expression]) extends Expression {
   override def eval(input: Row): Any = {
     var i = 0
     var result: Any = null
-    while(i < children.size && result == null) {
-      result = children(i).eval(input)
-      i += 1
+    val childIterator = children.iterator
+    while (childIterator.hasNext && result == null) {
+      result = childIterator.next().eval(input)
     }
     result
   }