fix(module:select): allow using arrow buttons on empty list

[Bonelol]

Prevent the error Cannot read property 'nzValue' of undefined

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• [x] 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?

Using arrow button on empty list will throw an error "Cannot read property 'nzValue' of undefined"

Issue Number: N/A

What is the new behavior?

Select component will not throw the error

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[zorro-bot[bot]]

This preview will be available after the AzureCI is passed.

[codecov[bot]]

Codecov Report

Merging #6692 (8914f70) into master (f9e7d23) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6692      +/-   ##
==========================================
+ Coverage   89.92%   89.96%   +0.03%     
==========================================
  Files         481      481              
  Lines       15403    15403              
  Branches     2492     2494       +2     
==========================================
+ Hits        13851    13857       +6     
+ Misses        931      929       -2     
+ Partials      621      617       -4     

Impacted Files	Coverage Δ
components/select/select.component.ts	87.45% <100.00%> (+0.32%)	:arrow_up:
components/tabs/tab-nav-bar.component.ts	83.09% <0.00%> (+0.70%)	:arrow_up:
components/time-picker/time-holder.ts	95.78% <0.00%> (+1.05%)	:arrow_up:
components/core/util/text-measure.ts	91.30% <0.00%> (+1.73%)	:arrow_up:

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[Bonelol]

#7018