code: private void writeHandleXMINillableAttribute(PrintWriter out, String indent, ModelAttribute attribute, String object) {
          ModelMethod method = attribute.method;
          String fieldName = method.field.name;
          String typeName = method.getType().getJavaImplTypeName();
          out.append(indent).append("            ").append(typeName).append(' ').append(fieldName).append(" = new ").append(typeName).append("();").println();
          out.append(indent).append("            parser.parse(").append(fieldName).append(");").println();
          out.append(indent).append("            if (!").append(fieldName).append(".isNil()) {").println();
          out.append(indent).append("                ").append(object).append('.').append(fieldName).append(" = ").append(fieldName).append(';').println();
          out.append(indent).append("            }").println();
          out.append(indent).append("            return true;").println();
      }
docs: Handle a nillable XMI attribute as an element with an xsi:nil="true".