[0.7.0-beta.1] tag: nzColor 应该允空字符串

[cipchk]

重现

<nz-tag>Tag 1</nz-tag>
<nz-tag [nzColor]="''">Tag 1</nz-tag>

https://stackblitz.com/edit/angular-kn9rqm?file=app/app.component.ts

理由

一般 tag 受控于数据的 color 值，在模板必须 nzColor 的前提下，无法呈现默认颜色 。

[ng-zorro-bot]

Hello @cipchk, your issue has been closed because it does not conform to our issue requirements. Please use the Issue Helper to create an issue, thank you!

[ng-zorro-bot]

It will be better to write your issue/comment in English, so more people can understand you.
And this means that more people can help you or benefit from your issue/comment.

[lock]

This thread has been automatically locked because it has not had recent activity. Please open a new issue for related bugs and link to relevant comments in this thread.