fix(CheckTreePicker): fix root node style incorrect

[superman66]

Fix CheckTreePicker style incorrect when data has only one level structure.

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[codecov]

Codecov Report

Merging #2279 (964f81e) into main (bd0f5e3) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2279      +/-   ##
==========================================
+ Coverage   85.64%   85.67%   +0.02%     
==========================================
  Files         274      274              
  Lines        8578     8578              
  Branches     2381     2381              
==========================================
+ Hits         7347     7349       +2     
+ Misses        667      666       -1     
+ Partials      564      563       -1     

Flag	Coverage Δ
ChromeCi	85.64% <100.00%> (+0.02%)	⬆️
Firefox	85.64% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
src/CheckTreePicker/CheckTreePicker.tsx	87.69% <ø> (ø)
src/CheckTreePicker/utils.ts	81.94% <100.00%> (+1.38%)	⬆️
src/CheckTreePicker/CheckTreeNode.tsx	64.28% <0.00%> (+2.38%)	⬆️

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