fix(module:avatar): Prioritize icons and text

[Eusen]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

1. avatar img 非正方形时会被压缩
2. 同时存在text与icon时，两者都会被显示出来
   Issue Number: N/A

What is the new behavior?

利用 object-fit: cover 解决了 avatar 图片被压缩的问题
提高了 text 的优先级，同时存在 icon 和 text，会优先显示text

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[netlify]

Deploy preview for ng-zorro-master ready!

Built with commit bf616c0

https://deploy-preview-4138--ng-zorro-master.netlify.com

[hsuanxyz]

@Eusen NG-ZORRO 的样式应该与 https://github.com/ant-design/ant-design 保持同步，你可以向他们提交 PR，合并后我们将自动同步样式

[Eusen]

@Eusen NG-ZORRO 的样式应该与 https://github.com/ant-design/ant-design 保持同步，你可以向他们提交 PR，合并后我们将自动同步样式

那这里的修改我给他删掉？

[codecov]

Codecov Report

Merging #4138 into master will decrease coverage by 0.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #4138      +/-   ##
==========================================
- Coverage   92.27%   92.25%   -0.02%     
==========================================
  Files         517      517              
  Lines       10961    10961              
  Branches     1985     1985              
==========================================
- Hits        10114    10112       -2     
- Misses        421      422       +1     
- Partials      426      427       +1

Impacted Files	Coverage Δ
components/avatar/nz-avatar.component.ts	95.31% <50%> (-3.13%)	⬇️

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[hsuanxyz]

@Eusen Hi，你需要 Rebase 一下 Master 代码，我们刚才修复了一个 CI 的问题

[Eusen]

@Eusen Hi，你需要 Rebase 一下 Master 代码，我们刚才修复了一个 CI 的问题

了解

[vthinkxie]

Hi @Eusen 请 rebase onto master ，否则 ci 会有问题

[Eusen]

恕在下git玩的不是很6，确实不知道该怎么操作。。
前段时间尝试过，但是没什么反应，rebase不到东西。