Null check for hazelcastInstance

Previously if the hazelcastInstance failed to start we would get a
NPE when the tests ended. This would mask the original issue of starting
the hazelcastInstance.

This commit ensures that hazelcastInstance is non-null before trying to
shut it down.

Polish gh-360
Relates to gh-360

spring-session/src/integration-test/java/org/springframework/session/hazelcast/HazelcastClientRepositoryITests.java

@@ -59,7 +59,9 @@ public class HazelcastClientRepositoryITests<S extends ExpiringSession>
 
 	@AfterClass
 	public static void teardown() {
-		hazelcastInstance.shutdown();
+		if(hazelcastInstance != null) {
+			hazelcastInstance.shutdown();
+		}
 	}
 
 	@Configuration