docs: public static ReferrerCounts create( Map<NodeKey, Integer> strongCountsByReferrerKey,
                                           Map<NodeKey, Integer> weakCountsByReferrerKey ) {
          if (strongCountsByReferrerKey == null) strongCountsByReferrerKey = EMPTY_COUNTS;
          if (weakCountsByReferrerKey == null) weakCountsByReferrerKey = EMPTY_COUNTS;
          if (strongCountsByReferrerKey.isEmpty() && weakCountsByReferrerKey.isEmpty()) return null;
          return new ReferrerCounts(strongCountsByReferrerKey, weakCountsByReferrerKey);
      }