IllegalArgumentException in statistics manager getGeneratedUUIDForNode() call

The current code meant to avoid NPE for the getGeneratedUUIDForNode() call leads to IllegalArgumentException because the UUID class is unhappy with string "invalid-uuid". So, might as well return null instead of doing bad workaround.

Following exception was thrown on my screen.

Exception in thread "pool-30-thread-1" java.lang.RuntimeException: java.lang.IllegalArgumentException: Invalid UUID string: invalid-uuid
at com.lmax.disruptor.FatalExceptionHandler.handleEventException(FatalExceptionHandler.java:45)
at com.lmax.disruptor.BatchEventProcessor.run(BatchEventProcessor.java:147)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IllegalArgumentException: Invalid UUID string: invalid-uuid
at java.util.UUID.fromString(UUID.java:194)
at org.opendaylight.openflowplugin.applications.statistics.manager.impl.StatisticsManagerImpl.getGeneratedUUIDForNode(StatisticsManagerImpl.java:399)
at org.opendaylight.openflowplugin.applications.statistics.manager.impl.StatListenCommitGroup$2.generatedUUIDForNode(StatListenCommitGroup.java:206)
at org.opendaylight.openflowplugin.applications.statistics.manager.StatisticsManager$StatDataStoreOperation.<init>(StatisticsManager.java:85)
at org.opendaylight.openflowplugin.applications.statistics.manager.impl.StatListenCommitGroup$2.<init>(StatListenCommitGroup.java:167)
at org.opendaylight.openflowplugin.applications.statistics.manager.impl.StatListenCommitGroup.onGroupFeaturesUpdated(StatListenCommitGroup.java:167)
at org.opendaylight.yangtools.yang.binding.util.NotificationListenerInvoker.invokeNotification(NotificationListenerInvoker.java:91)
at org.opendaylight.controller.md.sal.binding.impl.BindingDOMNotificationListenerAdapter.onNotification(BindingDOMNotificationListenerAdapter.java:44)
at org.opendaylight.controller.md.sal.dom.broker.impl.DOMNotificationRouterEvent.deliverNotification(DOMNotificationRouterEvent.java:50)
at org.opendaylight.controller.md.sal.dom.broker.impl.DOMNotificationRouter$1.onEvent(DOMNotificationRouter.java:68)
at org.opendaylight.controller.md.sal.dom.broker.impl.DOMNotificationRouter$1.onEvent(DOMNotificationRouter.java:65)
at com.lmax.disruptor.BatchEventProcessor.run(BatchEventProcessor.java:128)
... 3 more
Exception in thread "Thread-1216" java.util.concurrent.RejectedExecutionException: Task org.opendaylight.openflowplugin.openflow.md.core.HandshakeStepWrapper@10e0fdcd rejected from org.opendaylight.openflowplugin.openflow.md.core.ThreadPoolLoggingExecutor@14df6b09[Shutting down, pool size = 0, active threads = 0, queued tasks = 1, completed tasks = 0]
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2047)
at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:823)
at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1369)
at org.opendaylight.openflowplugin.openflow.md.core.ConnectionConductorImpl.onConnectionReady(ConnectionConductorImpl.java:456)
at org.opendaylight.openflowjava.protocol.impl.core.connection.ConnectionAdapterImpl$3.run(ConnectionAdapterImpl.java:472)
at java.lang.Thread.run(Thread.java:745)