fix(module:tree-select): should not close when the selectable is false

[hsuanxyz]

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: #3833

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[codecov]

Codecov Report

Merging #3843 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3843      +/-   ##
==========================================
+ Coverage   95.36%   95.36%   +<.01%     
==========================================
  Files         723      723              
  Lines       14722    14729       +7     
  Branches     1937     1937              
==========================================
+ Hits        14040    14047       +7     
  Misses        249      249              
  Partials      433      433

Impacted Files	Coverage Δ
components/tree-select/nz-tree-select.component.ts	99.08% <100%> (+0.03%)	⬆️

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