Fix possible null pointer in acumulator toString

Author: Michael Armbrust <michael@databricks.com>

Closes #1204 from marmbrus/nullPointerToString and squashes the following commits:

35b5fce [Michael Armbrust] Fix possible null pointer in acumulator toString

core/src/main/scala/org/apache/spark/Accumulators.scala

@@ -127,7 +127,7 @@ class Accumulable[R, T] (
     Accumulators.register(this, false)
   }
 
-  override def toString = value_.toString
+  override def toString = if (value_ == null) "null" else value_.toString
 }
 
 /**