code: public ClassConfiguration<T> merge(ClassConfiguration<T> other) {
          if (!targetClass.isAssignableFrom(other.targetClass)) {
              throw new IllegalArgumentException(
                      "Cannot merge class configurations: " + targetClass.getName() + " is not assignable to " + other
                              .targetClass.getName());
          }
          map.putAll(other.map);
          map.values().removeIf(Objects::isNull);
          return this;
      }
docs: Merge the class configuration with another one, overriding the existing values having an identical key. If a key
  with a null value is merged, the key is completely removed from the resulting configuration.
  
  @param other the other class configuration.
  @return the merge class configuration.