fix(module:tree-select): position error when in the modal-box

[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: #1681

What is the new behavior?

reposition when open.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #1687 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1687      +/-   ##
==========================================
+ Coverage   96.01%   96.02%   +<.01%     
==========================================
  Files         466      466              
  Lines       11091    11093       +2     
  Branches     1462     1462              
==========================================
+ Hits        10649    10652       +3     
  Misses        125      125              
+ Partials      317      316       -1

Impacted Files	Coverage Δ
components/tree-select/nz-tree-select.component.ts	99.03% <100%> (+0.49%)	⬆️

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