fix(vant-compat): [Field] Fix the false positive empty value when the value is boolean false

[YujiaCheng1996]

Fix the false positive empty value when the value is boolean false.
当值为布尔值的false时错误认为空值。
eg.

<van-radio-group v-model="checked">
  <van-radio :name="true">是</van-radio>
  <van-radio :name="false">否</van-radio>
</van-radio-group>

[chenjiahan]

This will break the Checkbox component

[YujiaCheng1996]

When a single boolean Radio/Checkbox/Switch is required, users have to select true, which is not ideal.
单选/多选/开关如果单独绑定布尔值并设置必填，用户必须选择是，若选否/关闭开关，校验不通过，不是很理想。
Users should be able to choose false, or turn off the switch. In this case, just check the value is undefined/null/NaN or not will be enough.
用户应该能选否或关闭开关，这样只要检查值不为空就行。
Maybe there is a more elegant apporach.
可能有其他更好的解决方法。

[chenjiahan]

嗯，修复这个问题的前提是不破坏现有的正确逻辑，比如 Checkbox 组件