fix(module: tree): nz-tree-drop-indicator for custom tree node templates

[Zarlex]

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?

The nz-tree-drop-indicator is only rendered for tree nodes if no custom treeTemplate is provided

Issue Number: N/A

What is the new behavior?

The nz-tree-drop-indicator is also rendered for tree nodes with a custom treeTemplate

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

In this screenshot you can see the new behaviour: https://www.awesomescreenshot.com/video/10188107
The first tree uses a custom node template, the other tree does not
In both trees you can now see the drop-indicator

The tree was taken from https://ng.ant.design/components/tree/en

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #7579 (610bcf6) into master (f18149a) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7579      +/-   ##
==========================================
- Coverage   91.74%   91.73%   -0.01%     
==========================================
  Files         502      502              
  Lines       16756    16756              
  Branches     2757     2757              
==========================================
- Hits        15372    15371       -1     
- Misses       1050     1051       +1     
  Partials      334      334              

Impacted Files	Coverage Δ
components/tree/tree-node-title.component.ts	100.00% <ø> (ø)
components/core/util/text-measure.ts	92.10% <0.00%> (-0.88%)	⬇️

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[behdi]

+1. Heavily needed feature.

[simplejason]

LGTM