kiji-schema: SCHEMA-866. AbstractCassandraKijiIntegrationTest should check for null before closing a Kiji instance

issue: https://jira.kiji.org/browse/SCHEMA-866
review: https://review.kiji.org/r/2034
commit: https://github.com/kijiproject/kiji-schema/commit/bb66534a4b7ae15aa323d13b83a8bf46ae0b8c58