protected Type getFieldType(int ordinal) {
        return sortedGetters[ordinal].fieldInfo.fieldType;
    }
Get field names of not null fields. Keep the same logic as getSize.

@param object the object to be checked
@return field name set
@throws Exception
@see #getSize(Object)
src/main/java/com/alibaba/fastjson/serializer/JavaBeanSerializer.java