fix bug when set indexer as db and add tests

[lunny]

caused by #6144

[codecov-io]

Codecov Report

Merging #6173 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6173      +/-   ##
==========================================
+ Coverage   38.83%   38.88%   +0.05%     
==========================================
  Files         354      354              
  Lines       50183    50184       +1     
==========================================
+ Hits        19490    19516      +26     
+ Misses      27868    27842      -26     
- Partials     2825     2826       +1

Impacted Files	Coverage Δ
modules/indexer/issues/indexer.go	65.45% <100%> (+5.82%)	⬆️
routers/repo/view.go	42.07% <0%> (+0.99%)	⬆️
models/unit.go	14.28% <0%> (+14.28%)	⬆️
modules/indexer/issues/db.go	59.09% <0%> (+59.09%)	⬆️

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