Ensure that titus server groups have non-null 'instances' collection

Avoids an NPE in ClusterController.

src/main/groovy/com/netflix/spinnaker/clouddriver/titus/caching/providers/TitusClusterProvider.groovy

@@ -217,6 +217,7 @@ class TitusClusterProvider implements ClusterProvider<TitusCluster> {
           new TitusInstance(it as Map)
         }
       }
+      serverGroup.instances = serverGroup.instances ?: []
 
       [(serverGroupEntry.id): serverGroup]
     }