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>