[API] make change repo settings work on empty repos

[6543]

close #15602

[codecov-commenter]

Codecov Report

Merging #15778 (b2956c7) into main (272bbb2) will decrease coverage by 0.00%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##             main   #15778      +/-   ##
==========================================
- Coverage   43.96%   43.95%   -0.01%     
==========================================
  Files         678      678              
  Lines       81999    81998       -1     
==========================================
- Hits        36048    36043       -5     
- Misses      40076    40077       +1     
- Partials     5875     5878       +3     

Impacted Files	Coverage Δ
routers/api/v1/repo/repo.go	57.99% <14.28%> (-0.34%)	⬇️
routers/api/v1/api.go	78.15% <100.00%> (ø)
models/unit.go	46.57% <0.00%> (-2.74%)	⬇️
modules/queue/workerpool.go	51.64% <0.00%> (-1.10%)	⬇️
routers/repo/view.go	40.66% <0.00%> (-0.59%)	⬇️
services/pull/pull.go	43.83% <0.00%> (+0.45%)	⬆️
modules/notification/mail/mail.go	41.83% <0.00%> (+3.06%)	⬆️

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[6543]

-> #15789