Fix NPE when CASSANDRA_CLUSTER_NODES not set

See the test failures here: http://brooklyn.builds.cloudsoftcorp.com/job/Brooklyn-Master-Build/172/

Test Result (3 failures / +3)
brooklyn.entity.nosql.cassandra.CassandraClusterTest.testPopulatesInitialSeeds
brooklyn.entity.nosql.cassandra.CassandraClusterTest.testPopulatesInitialTokens
brooklyn.entity.nosql.cassandra.CassandraClusterTest.testUpdatesSeedsOnFailuresAndAdditions