fix(module:slider): processing value zero correctly

[stygian-desolator]

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?

Issue Number: N/A

close #6635

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #6729 (f70da52) into master (f9e7d23) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6729      +/-   ##
==========================================
- Coverage   89.92%   89.91%   -0.02%     
==========================================
  Files         481      480       -1     
  Lines       15403    15362      -41     
  Branches     2492     2490       -2     
==========================================
- Hits        13851    13812      -39     
+ Misses        931      930       -1     
+ Partials      621      620       -1     

Impacted Files	Coverage Δ
components/slider/slider.component.ts	90.40% <100.00%> (ø)
components/date-picker/date-picker.component.ts	90.90% <0.00%> (-2.47%)	⬇️
...mponents/date-picker/date-range-popup.component.ts	90.21% <0.00%> (-0.21%)	⬇️
components/date-picker/date-picker.module.ts	100.00% <0.00%> (ø)
components/date-picker/picker.component.ts
components/tabs/tab-nav-bar.component.ts	83.09% <0.00%> (+0.70%)	⬆️
components/date-picker/testing/util.ts	100.00% <0.00%> (+22.22%)	⬆️

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[wzhudev]

LGTM

[hsuanxyz]

LGTM