fix(uploader): convert default slot to boolean value

[chouchouji]

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• Fix linting errors
• Tests have been added / updated (or snapshots)

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Name	Status	Preview	Comments	Updated (UTC)
varlet	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 22, 2023 4:07pm

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (842b10c) 93.09% compared to head (55dd1bd) 93.08%.
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1188      +/-   ##
==========================================
- Coverage   93.09%   93.08%   -0.01%     
==========================================
  Files         284      284              
  Lines        6252     6251       -1     
  Branches     1519     1518       -1     
==========================================
- Hits         5820     5819       -1     
  Misses        370      370              
  Partials       62       62              

Files Changed	Coverage Δ
packages/varlet-ui/src/uploader/Uploader.vue	94.11% <100.00%> (ø)

... and 1 file with indirect coverage changes

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