Fix notification page panic when add and remove comment

[jaqra]

This pr fixes 500 internal server error on notifications page when someone add issue comment then remove it immediately

[codecov-io]

Codecov Report

Merging #9337 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #9337      +/-   ##
==========================================
- Coverage   41.27%   41.26%   -0.02%     
==========================================
  Files         561      561              
  Lines       73439    73439              
==========================================
- Hits        30315    30305      -10     
- Misses      39332    39340       +8     
- Partials     3792     3794       +2

Impacted Files	Coverage Δ
models/notification.go	66.37% <0%> (ø)	⬆️
services/pull/check.go	55.63% <0%> (-2.26%)	⬇️
models/repo_list.go	73.14% <0%> (-0.93%)	⬇️
models/pull.go	54.4% <0%> (-0.5%)	⬇️
models/repo.go	47.97% <0%> (-0.1%)	⬇️

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[lafriks]

shouldn't notification just be removed on deleting comment?