Fix NPE in SimpleBuilders.ParititionUpdateBulder.RTBuilder.build

These two arrays are not initialised (1) which means that if I do not set start nor end, when ClusteringBound.create is called, it will be null, but it will fail here (2) as values will be null.

The fix consists of check if values are null and if they are, we build that bound immediately.

(1) https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/db/SimpleBuilders.java#L257-L258
(2) https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/db/ClusteringBound.java#L128