code: private Set<String> getNotNullExtraKeys(Node actual, Set<String> extraKeys) {
          Set<String> notNullExtraKeys = new TreeSet<>();
          for (String extraKey : extraKeys) {
              if (!actual.get(extraKey).isNull()) {
                  notNullExtraKeys.add(extraKey);
              }
          }
          return notNullExtraKeys;
      }
docs: Returns extra keys that are not null.