private static List<DocumentDatabase> getDocumentDbs(SearchCluster cluster) {
        if (cluster != null) {
            return cluster.getDocumentDbs();
        }
        return List.of();
    }
Validates the changes between all current and next indexed search clusters in a vespa model.

@author geirst
config-model/src/main/java/com/yahoo/vespa/model/application/validation/change/IndexedSearchClusterChangeValidator.java