Fix NullPointerException when importing data from early 1970

Hello, I have found that importing data with very data point timestamp close to 0 triggers NullPointerException (see below).

I tracked the problem in the branch introduced comparison between newRowTime and rowTime... We don;t enter the branch if the rowTime is zero.
This branch also instanciates the rowKey object, therefore I modified the condition to enter the branch if the rowKey object has not yet been instanciated.


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java.lang.NullPointerException
	at org.kairosdb.datastore.cassandra.DataPointsRowKeySerializer.toByteBuffer(DataPointsRowKeySerializer.java:32)
	at org.kairosdb.datastore.cassandra.DataPointsRowKeySerializer.toByteBuffer(DataPointsRowKeySerializer.java:24)
	at me.prettyprint.cassandra.service.BatchMutation.getInnerMutationMap(BatchMutation.java:131)
	at me.prettyprint.cassandra.service.BatchMutation.addMutation(BatchMutation.java:113)
	at me.prettyprint.cassandra.service.BatchMutation.addInsertion(BatchMutation.java:66)
	at me.prettyprint.cassandra.model.MutatorImpl.addInsertion(MutatorImpl.java:159)
	at org.kairosdb.datastore.cassandra.WriteBuffer.addData(WriteBuffer.java:84)
	at org.kairosdb.datastore.cassandra.CassandraDatastore.putDataPoints(CassandraDatastore.java:283)
	at org.kairosdb.core.datastore.KairosDatastore.putDataPoints(KairosDatastore.java:156)
	at org.kairosdb.core.Main.runImport(Main.java:332)
	at org.kairosdb.core.Main.main(Main.java:179)