API: make sure labels are actually returned

[adelowo]

This fixes an item in #6047 .

I couldn't replicate the other 401 the poster talked about

[codecov-io]

Codecov Report

Merging #6053 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #6053      +/-   ##
==========================================
- Coverage   38.87%   38.86%   -0.01%     
==========================================
  Files         345      345              
  Lines       49503    49507       +4     
==========================================
- Hits        19243    19241       -2     
- Misses      27477    27483       +6     
  Partials     2783     2783

Impacted Files	Coverage Δ
routers/api/v1/repo/issue_label.go	29.52% <0%> (-0.45%)	⬇️
models/repo_list.go	63.29% <0%> (-1.27%)	⬇️

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[zeripath]

I don't think there's a test covering this API, although I understand you can't quite reproduce the issue - it would be good if there was at least a test.