Fix for a NullPointerException when hostname is null in Task Metrics

If the hostname is null in the Spark Task Metrics, this causes the following NullPointerException:

java.lang.NullPointerException
	at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:191)
	at com.google.common.collect.MapMakerInternalMap.putIfAbsent(MapMakerInternalMap.java:3507)
	at com.google.common.collect.Interners$WeakInterner.intern(Interners.java:85)
	at com.netflix.servo.tag.Tags.intern(Tags.java:33)
	at com.netflix.servo.tag.Tags.newTag(Tags.java:53)
	at org.bdgenomics.adam.instrumentation.TaskTimer$$anonfun$$plus$eq$1.apply(SparkMetrics.scala:184)
	at org.bdgenomics.adam.instrumentation.TaskTimer$$anonfun$$plus$eq$1.apply(SparkMetrics.scala:184)
	at scala.collection.mutable.MapLike$class.getOrElseUpdate(MapLike.scala:189)
	at scala.collection.mutable.AbstractMap.getOrElseUpdate(Map.scala:91)
	at org.bdgenomics.adam.instrumentation.TaskTimer.$plus$eq(SparkMetrics.scala:183)
	at org.bdgenomics.adam.instrumentation.ADAMMetricsListener.onTaskEnd(ADAMMetricsListener.scala:48)
<stack trace truncated>

It's not clear under what circumstances this happens, but I've seen it when a job was cancelled part-way through.