fix(module:time-picker): add null input judgement when using datefns

[wenqi73]

Close #3854

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

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[codecov]

Codecov Report

Merging #4283 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4283      +/-   ##
==========================================
- Coverage   92.18%   92.17%   -0.01%     
==========================================
  Files         520      520              
  Lines       11005    11005              
  Branches     1995     1996       +1     
==========================================
- Hits        10145    10144       -1     
  Misses        429      429              
- Partials      431      432       +1

Impacted Files	Coverage Δ
components/i18n/date-helper.service.ts	68% <0%> (-4%)	⬇️

---

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 b1bba9e...da6e1f0. Read the comment docs.

[netlify]

Deploy preview for ng-zorro-master ready!

Built with commit da6e1f0

https://deploy-preview-4283--ng-zorro-master.netlify.com

[vthinkxie]

LGTM