fix nil pointer when adding a due date

[adelowo]

I was trying to pull #5568 to test it locally. But on trying to add a due date,
I kept on getting "The due date is invalid or out of range. Please use the format 'yyyy-mm-dd'."
Looked at my terminal and it was a panic.

Commit I am running locally : a06ae0b

Log --> https://gist.github.com/adelowo/d74a006e9d77d6be6fa41a50a5e09008

[codecov-io]

Codecov Report

Merging #5587 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #5587      +/-   ##
==========================================
+ Coverage   37.52%   37.52%   +<.01%     
==========================================
  Files         322      322              
  Lines       47283    47286       +3     
==========================================
+ Hits        17741    17746       +5     
+ Misses      26992    26988       -4     
- Partials     2550     2552       +2

Impacted Files	Coverage Δ
models/issue_comment.go	46.6% <0%> (-0.18%)	⬇️
modules/process/manager.go	76.81% <0%> (-4.35%)	⬇️
routers/repo/view.go	46.01% <0%> (+1.22%)	⬆️
models/repo_list.go	64.55% <0%> (+1.26%)	⬆️
models/unit.go	14.28% <0%> (+14.28%)	⬆️

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[rvillablanca]

Changes have been made so is possible to merge this PR ?

[zeripath]

@rvillablanca - I think @lunny needs to accept your changes. He's on Chinese time (I think) (GMT+8) it's not quite 6am there currently.

[rvillablanca]

Thanks @zeripath, these are not my changes but I need them to make my changes over master branch and not over v1.6.2 tag.

[lafriks]

@lunny need your approval