public static List<SearchCluster> getSearchClusters(ConfigModelRepo pc) {
        List<SearchCluster> clusters = new ArrayList<>();
        for (ContentCluster c : getContentClusters(pc)) {
            SearchCluster sc = c.getSearch().getSearchCluster();
            if (sc != null) {
                clusters.add(sc);
            }
        }
        return clusters;
    }
Returns indexing cluster implicitly created by this,
or empty if an explicit cluster is used (or if called before the build phase)
config-model/src/main/java/com/yahoo/vespa/model/content/Content.java