Make repo creation for API similar to UI

[jolheiser]

Currently when creating a repo via the API and using the auto_init key while leaving the readme key blank, Gitea returns a 500 error Asset readme not found

This PR checks if auto_init was set to true and readme is left blank, set readme to Default.
On the UI, readme is defaulted as Default which is why the UI never encounters this error.

If it would make more sense to handle this "defaultness" in the model, I can move it there. I only added it to the API to mirror what the UI does.

One issue I can think of is if someone builds Gitea from source without bindata and removed the Default readme, this would no longer work. Not sure if that's in the scope of this PR though...

[codecov-io]

Codecov Report

Merging #6142 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #6142      +/-   ##
==========================================
+ Coverage   38.82%   38.83%   +<.01%     
==========================================
  Files         354      354              
  Lines       50171    50174       +3     
==========================================
+ Hits        19477    19483       +6     
+ Misses      27873    27868       -5     
- Partials     2821     2823       +2

Impacted Files	Coverage Δ
routers/api/v1/repo/repo.go	55.82% <0%> (-0.35%)	⬇️
routers/repo/view.go	42.07% <0%> (+0.99%)	⬆️
models/unit.go	14.28% <0%> (+14.28%)	⬆️

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