public void writeObjectNull(Class<?> fieldClass) {
        if ((this.context.features & (MASK_NULL_AS_DEFAULT_VALUE)) != 0) {
            if (fieldClass == Character.class) {
                writeString("\u0000");
            } else {
                writeRaw('{', '}');
            }
        } else {
            writeNull();
        }
    }
If ReferenceDetection has been set, returns the path of the current object, otherwise returns null
@since 2.0.51
@return the path of the current object
core/src/main/java/com/alibaba/fastjson2/JSONWriter.java