Protect from null env variables in mesos.

core/src/main/scala/spark/scheduler/mesos/CoarseMesosSchedulerBackend.scala

@@ -114,11 +114,15 @@ private[spark] class CoarseMesosSchedulerBackend(
     val command = "\"%s\" spark.executor.StandaloneExecutorBackend %s %s %s %d".format(
       runScript, masterUrl, offer.getSlaveId.getValue, offer.getHostname, numCores)
     val environment = Environment.newBuilder()
-    sc.executorEnvs.foreach { case(k,v) => 
-      environment.addVariables(Environment.Variable.newBuilder()
-          .setName(k)
-          .setValue(v)
+    sc.executorEnvs.foreach { case(key, value) =>
+      if (value == null) {
+        logInfo("Environment variable not set: " + key)
+      } else {
+        environment.addVariables(Environment.Variable.newBuilder()
+          .setName(key)
+          .setValue(value)
           .build())
+      }   
     }
     return CommandInfo.newBuilder().setValue(command).setEnvironment(environment).build()
   }

core/src/main/scala/spark/scheduler/mesos/MesosSchedulerBackend.scala

@@ -85,11 +85,15 @@ private[spark] class MesosSchedulerBackend(
     }
     val execScript = new File(sparkHome, "spark-executor").getCanonicalPath
     val environment = Environment.newBuilder()
-    sc.executorEnvs.foreach { case(k,v) => 
-      environment.addVariables(Environment.Variable.newBuilder()
-          .setName(k)
-          .setValue(v)
+    sc.executorEnvs.foreach { case(key, value) =>
+      if (value == null) {
+        logInfo("Environment variable not set: " + key)
+      } else {
+        environment.addVariables(Environment.Variable.newBuilder()
+          .setName(key)
+          .setValue(value)
           .build())
+      }   
     }
     val memory = Resource.newBuilder()
       .setName("mem")