fix(DateTimePicker): resolved the initial value cannot be empty

[anlyyao]

🤔 这个 PR 的性质是？

• [x] 日常 bug 修复
• [ ] 新特性提交
• [ ] 文档改进
• [ ] 演示代码改进
• [x] 组件样式/交互改进
• [ ] CI/CD 改进
• [ ] 重构
• [ ] 代码风格优化
• [ ] 测试用例
• [ ] 分支合并
• [ ] 其他

🔗 相关 Issue

fix #767 fix #766

💡 需求背景和解决方案

背景：时间选择器初始值不能为空的问题，官网Form组件可复现。

📝 更新日志

• break(DateTimePicker): 调整DOM结构

• fix(DateTimePicker): 修复初始值不能为空的问题

• fix(DateTimePicker): 修复选中文本和弹窗面板圆角样式错误

• fix(Picker): 修复选中文本样式错误

• [ ] 本条 PR 不需要纳入 Changelog

☑️ 请求合并前的自查清单

⚠️ 请自检并全部勾选全部选项。⚠️

• [ ] 文档已补充或无须补充
• [ ] 代码演示已提供或无须提供
• [ ] TypeScript 定义已补充或无须补充
• [ ] Changelog 已提供或无须提供

[github-actions[bot]]

[github-actions[bot]]

Size Change: +171 B (0%)

Total Size: 470 kB

Filename	Size	Change
./dist/tdesign.css	35.4 kB	+33 B (0%)
./dist/tdesign.js	276 kB	+72 B (0%)
./dist/tdesign.min.css	25.6 kB	+31 B (0%)
./dist/tdesign.min.js	133 kB	+35 B (0%)

[github-actions[bot]]

Size Change: +171 B (0%)

Total Size: 470 kB

Filename	Size	Change
./dist/tdesign.css	35.4 kB	+33 B (0%)
./dist/tdesign.js	276 kB	+72 B (0%)
./dist/tdesign.min.css	25.6 kB	+31 B (0%)
./dist/tdesign.min.js	133 kB	+35 B (0%)

[LeeJim]

DOM 结构也发生了变化

[anlyyao]

DOM 结构也发生了变化

已补充日志

[github-actions[bot]]

Size Change: +102 B (0%)

Total Size: 470 kB

Filename	Size	Change
./dist/tdesign.css	35.4 kB	+34 B (0%)
./dist/tdesign.js	276 kB	+12 B (0%)
./dist/tdesign.min.css	25.7 kB	+33 B (0%)
./dist/tdesign.min.js	133 kB	+23 B (0%)

[github-actions[bot]]

Size Change: +161 B (0%)

Total Size: 470 kB

Filename	Size	Change
./dist/tdesign.css	35.4 kB	+34 B (0%)
./dist/tdesign.js	276 kB	+59 B (0%)
./dist/tdesign.min.css	25.7 kB	+33 B (0%)
./dist/tdesign.min.js	133 kB	+35 B (0%)

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `./dist/reset.css` | 672 B |