fix(form): validate 获取子节点规则方式调整

[eiinu]

这个 PR 做了什么? (简要描述所做更改)

这个 PR 是什么类型? (至少选择一个)

• 新特性提交
• 日常 bug 修复
• 站点、文档改进
• 演示代码改进
• 组件样式/交互改进
• TypeScript 定义更新
• 包体积优化
• 性能优化
• 功能增强
• 国际化改进
• 代码重构
• 代码风格优化
• 测试用例
• 分支合并
• 其他改动（是关于什么的改动？）

这个 PR 涉及以下平台:

• NutUI 4.0 H5
• NutUI 4.0 小程序
• NutUI 3.0 H5
• NutUI 3.0 小程序

这个 PR 是否已自测:

• 自测 Vite 脚手架使用 测试仓库
• 自测 Taro 脚手架使用小程序 & Taro-H5 测试仓库

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (326b893) 84.89% compared to head (9ee063c) 84.93%.

❗ Current head 9ee063c differs from pull request most recent head 78ad30b. Consider uploading reports for the commit 78ad30b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##               v4    #2593      +/-   ##
==========================================
+ Coverage   84.89%   84.93%   +0.03%     
==========================================
  Files         150      150              
  Lines       20589    20620      +31     
  Branches     2302     2297       -5     
==========================================
+ Hits        17480    17514      +34     
+ Misses       3104     3101       -3     
  Partials        5        5              

Files	Coverage Δ
src/packages/__VUE/form/common.ts	84.68% <100.00%> (+3.15%)	⬆️
src/packages/__VUE/formitem/index.vue	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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