[Bug Report] Field type="date" 没有值时在手机端不显示内容

### Mint UI version
2.2.11

### OS/Browsers version
Android/iOS

### Vue version
2.5.9

### Reproduction Link
[https://elemefe.github.io/mint-ui/#/field](https://elemefe.github.io/mint-ui/#/field)

### Steps to reproduce
Field type="date" 没有值时在手机端不显示内容

### What is Expected?
期待默认显示placeholder的值

### What is actually happening?
在移动端默认什么都不显示

Start with the Field reproduction link and verify the empty type="date" behavior on Android and iOS using Vue 2.5.9. Compare the mobile result with the expected placeholder display, and consider the issue complete when an empty date field shows its placeholder consistently on both platforms.

Written by the indexing model from the issue text.