fix(area): columns-placeholder prop rendering error

[yipl95]

Before submitting a pull request, please read the contributing guide.

在提交 pull request 之前，请阅读 贡献指南。

bugfix #12856

当传入 placeholder 后，处理 市、区 数据有点问题，尝试如下修改：

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.76%. Comparing base (8cb3fe5) to head (a3e464f).
Report is 5 commits behind head on main.

❗ Current head a3e464f differs from pull request most recent head e825a1f

Please upload reports for the commit e825a1f to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12857      +/-   ##
==========================================
+ Coverage   89.67%   89.76%   +0.08%     
==========================================
  Files         257      257              
  Lines        6957     6957              
  Branches     1711     1711              
==========================================
+ Hits         6239     6245       +6     
+ Misses        382      379       -3     
+ Partials      336      333       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[inottn]

可以补充一下对应的单测

[yipl95]

@inottn 已添加单测，测试结果如下：

单测运行结果：

[inottn]

@inottn 已添加单测，测试结果如下：

单测运行结果：

嗯，这个是 demo 的快照测试，没有覆盖到本次 PR 对应的 issue。可以试着编写一个单测，确保它在这个 PR 前运行失败，在这个 PR 后运行成功。

[yipl95]

get~ 我试下

[yipl95]

@inottn 大佬看下是否符合预期，新增单测用例如下：

test('should columns placeholder displayed normally', async () => {
  const columnsPlaceholder = ['省份', '城市', '区县']

  const wrapper = mount(Area, {
    props: {
      areaList,
      columnsPlaceholder,
    },
  });

  const columns = await wrapper.findAll('.van-picker-column');
  // 点击省份（第一列第二行）
  await columns[0].findAll('li')?.[1].trigger('click');
  // 点击后获取第二列第一行内容，判断是否 === columnsPlaceholder[1] => '城市'
  expect(columns[1].find('li').text()).toBe(columnsPlaceholder[1])

  // 点击城市（第二列第二行）
  await columns[1].findAll('li')?.[1].trigger('click');
  // 点击后获取第三列第一行内容，判断是否 === columnsPlaceholder[2] => '区县'
  expect(columns[2].find('li').text()).toBe(columnsPlaceholder[2])
});

修改前：

修改后：

[chenjiahan]

Thanks! ❤️