如何使用itemloader填充空项？

from scrapy.loader import ItemLoader
from scrapy import Item

class MyItem(Item):
    field1 = scrapy.Field()
    field2 = scrapy.Field()
    # 其他字段...

from scrapy import Spider

class MySpider(Spider):
    name = 'my_spider'
    start_urls = ['http://example.com']

    def parse(self, response):
        loader = ItemLoader(item=MyItem(), response=response)
        loader.add_xpath('field1', '//div[@class="field1"]/text()')
        loader.add_xpath('field2', '//div[@class="field2"]/text()')
        # 添加其他字段的XPath表达式...

        item = loader.load_item()
        yield item

ITEM_PIPELINES = {
    'myproject.pipelines.MyPipeline': 300,
}

from scrapy.exceptions import DropItem

class MyPipeline(object):
    def process_item(self, item, spider):
        if not item['field1']:
            item['field1'] = '默认值1'
        if not item['field2']:
            item['field2'] = '默认值2'
        # 处理其他字段的空项...

        return item