Merge pull request apache#330 from tgravescs/fix_addjars_null_handling

Fix handling of empty SPARK_EXAMPLES_JAR

Currently if SPARK_EXAMPLES_JAR is left unset you get a null pointer exception when running the examples (atleast on spark on yarn).  The null now gets turned into a string of "null" when its put into the SparkConf so addJar no longer properly ignores it. This fixes that so that it can be left unset.

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

@@ -24,7 +24,7 @@ import com.typesafe.config.ConfigFactory
  *
  * @param loadDefaults whether to load values from the system properties and classpath
  */
-class SparkConf(loadDefaults: Boolean) extends Serializable with Cloneable {
+class SparkConf(loadDefaults: Boolean) extends Serializable with Cloneable with Logging {
 
   /** Create a SparkConf that loads defaults from system properties and the classpath */
   def this() = this(true)
@@ -67,7 +67,8 @@ class SparkConf(loadDefaults: Boolean) extends Serializable with Cloneable {
 
   /** Set JAR files to distribute to the cluster. */
   def setJars(jars: Seq[String]): SparkConf = {
-    set("spark.jars", jars.mkString(","))
+    for (jar <- jars if (jar == null)) logWarning("null jar passed to SparkContext constructor") 
+    set("spark.jars", jars.filter(_ != null).mkString(","))
   }
 
   /** Set JAR files to distribute to the cluster. (Java-friendly version.) */