fix(module:slider): fix defaultValue check error

[wzhudev]

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?

[ ] 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

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[netlify]

Deploy preview for ng-zorro-master ready!

Built with commit 55171eb

https://deploy-preview-2986--ng-zorro-master.netlify.com

[codecov]

Codecov Report

Merging #2986 into master will decrease coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2986      +/-   ##
==========================================
- Coverage   97.35%   97.35%   -0.01%     
==========================================
  Files         548      548              
  Lines       11436    11435       -1     
  Branches      813      813              
==========================================
- Hits        11133    11132       -1     
  Misses        190      190              
  Partials      113      113

Impacted Files	Coverage Δ
components/slider/nz-slider-error.ts	50% <0%> (ø)	⬆️
components/slider/nz-slider.component.ts	94.03% <100%> (-0.03%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update daec0ac...55171eb. Read the comment docs.

[cipchk]

LGTM

[vthinkxie]

LGTM