feat(module:time-picker): support null value in time-picker-panel

[vthinkxie]

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)
[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
[ ] No

Other information

[codecov]

Codecov Report

Merging #1388 into master will decrease coverage by 0.1%.
The diff coverage is 73.33%.

@@            Coverage Diff             @@
##           master    #1388      +/-   ##
==========================================
- Coverage   97.04%   96.94%   -0.11%     
==========================================
  Files         196      196              
  Lines        8367     8370       +3     
  Branches     1106     1106              
==========================================
- Hits         8120     8114       -6     
- Misses         30       36       +6     
- Partials      217      220       +3

Impacted Files	Coverage Δ
...ents/time-picker/nz-time-picker-panel.component.ts	84.39% <72%> (-1.47%)	⬇️
components/time-picker/nz-time-picker.component.ts	89.79% <80%> (-6%)	⬇️

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