build: add strictNullCheck config

[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)
[x] 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

[wzhudev]

@wilsoncook @cipchk @vthinkxie @hsuanxyz @simplejason

Please review code of the components you are in charge of. In case of any state related bug introduced.

Notes for further development:

1. Pay attention to properties that can be set to null or undefined. Try to refactor again in milestone 2.
2. When you want to get a form field, you have to assert that you can get it. Like formGroup.get('name')!.hasError. In template you have to use formGroup.get('name')?.hasError.
3. When a function parameter is optional, pass undefined instead of null.
4. Pay attention to optional declaration of a property. ?:
5. Avoid to declare a property as someOtherType | null, ?:someOtherType is preferred.
6. For subscriptions that need to be unsubscribe when component destroys, ! is free to use.
7. In test, native API like querySelector may returns null. You can add an assertion.
8. Use helpers like Required<> Optional<>.

Modules acquire additional attention:

1. Date Picker, especially helper class CandyDate.
2. Upload, especially relate to File.
3. Tree.

[cipchk]

我提了几处语法建议，我不确定这些变更是否可能会影响到一些测试用例。

[wzhudev]

@simplejason 我注意到 nz-tree-base 的 formatEvent 方法返回的 NzFormatEmitEvent 被各个 subject 用来 emit 事件, 但是不同 subject emit 的事件对于字段的 optional 要求是不一样的, 我添加了一些 Required (typescript 的类型修饰符), 请检查一下会不会有问题.

另外请加查一下 NzTreeNode 各个字段是否是必须的, 在不带来 break change 的前提下, ! 用的少一些就还是好一些.

[netlify]

Deploy preview for ng-zorro-master ready!

Built with commit 26a91f6

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[netlify]

Deploy preview for ng-zorro-master ready!

Built with commit 03aa3bf

https://deploy-preview-2977--ng-zorro-master.netlify.com

[codecov]

Codecov Report

Merging #2977 into master will increase coverage by <.01%.
The diff coverage is 95.53%.

@@            Coverage Diff             @@
##           master    #2977      +/-   ##
==========================================
+ Coverage   97.32%   97.32%   +<.01%     
==========================================
  Files         549      550       +1     
  Lines       11489    11495       +6     
  Branches      815      817       +2     
==========================================
+ Hits        11182    11188       +6     
- Misses        197      199       +2     
+ Partials      110      108       -2

Impacted Files	Coverage Δ
...s/date-picker/lib/decade/decade-panel.component.ts	100% <ø> (ø)	⬆️
components/select/nz-option.pipe.ts	100% <ø> (ø)	⬆️
components/slider/nz-slider-definitions.ts	100% <ø> (ø)	⬆️
components/slider/nz-slider-track.component.ts	100% <ø> (ø)	⬆️
...nts/date-picker/lib/month/month-table.component.ts	100% <ø> (ø)	⬆️
components/carousel/nz-carousel.component.ts	93.54% <ø> (ø)	⬆️
...nents/date-picker/lib/year/year-panel.component.ts	100% <ø> (ø)	⬆️
components/message/nz-message.service.ts	97.91% <ø> (ø)	⬆️
components/dropdown/nz-dropdown.service.ts	100% <ø> (ø)	⬆️
...mponents/notification/nz-notification.component.ts	100% <ø> (ø)	⬆️
... and 85 more

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