fix(module:timeline): fix reverse bug

[wzhudev]

close #4509

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)
[x] 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?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #4690 into master will increase coverage by 0.01%.
The diff coverage is 98.24%.

@@            Coverage Diff             @@
##           master    #4690      +/-   ##
==========================================
+ Coverage   93.18%   93.19%   +0.01%     
==========================================
  Files         501      501              
  Lines       12750    12734      -16     
  Branches     1957     1955       -2     
==========================================
- Hits        11881    11868      -13     
  Misses        457      457              
+ Partials      412      409       -3

Impacted Files	Coverage Δ
components/timeline/timeline.module.ts	100% <ø> (ø)
components/core/util/dom.ts	76.92% <ø> (-4.03%)	⬇️
components/timeline/timeline-item.component.ts	100% <100%> (ø)
components/timeline/timeline.component.ts	97.5% <97.5%> (ø)

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[hsuanxyz]

@wendellhu95 Rebase 一下

[wzhudev]

ping @hsuanxyz