code: public static JsonNode.Type getMapValueType(JsonNode schema) {
          JsonNode.Type type = JsonNode.Type.valueOf(schema.get("type").asString().toUpperCase());
          JsonNode child;
          if (type == JsonNode.Type.OBJECT && (child = schema.get("additionalProperties")) != null) {
              return getFirstNonArrayValueType(child);
          }
          return null;
      }
docs: Returns the type of the elements of map (an object with
  additionalProperties of type specified) schema.
  
  @param schema
  @return null if is not map schema