cruise-control: Metric Anomaly Detector NPE

After rebuilding off of head for the fixes in #217 , I see these in the logs a lot.

[2018-05-07 13:38:55,817] WARN Metric Anomaly Detector encountered exception: (com.linkedin.kafka.cruisecontrol.detector.MetricAnomalyDetector) java.lang.NullPointerException at java.util.Collections$UnmodifiableCollection.<init>(Collections.java:1026) at java.util.Collections$UnmodifiableList.<init>(Collections.java:1302) at java.util.Collections.unmodifiableList(Collections.java:1287) at com.linkedin.cruisecontrol.monitor.sampling.aggregator.ValuesAndExtrapolations.windows(ValuesAndExtrapolations.java:63) at com.linkedin.cruisecontrol.detector.metricanomaly.PercentileMetricAnomalyFinder.metricAnomalies(PercentileMetricAnomalyFinder.java:136) at com.linkedin.kafka.cruisecontrol.detector.MetricAnomalyDetector.run(MetricAnomalyDetector.java:65) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308) at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(ScheduledThreadPoolExecutor.java:180) at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:294) 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:748)