Fix JSON result of empty array in heatmap data array

[sapk]

Fix #5151 by init the array so that the JSON translation is not null but [].

• some test

[codecov-io]

Codecov Report

Merging #5154 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5154      +/-   ##
==========================================
- Coverage   37.49%   37.48%   -0.01%     
==========================================
  Files         309      309              
  Lines       45782    45783       +1     
==========================================
- Hits        17164    17162       -2     
  Misses      26157    26157              
- Partials     2461     2464       +3

Impacted Files	Coverage Δ
models/user_heatmap.go	80% <100%> (+1.05%)	⬆️
modules/process/manager.go	76.81% <0%> (-4.35%)	⬇️
models/repo_indexer.go	50.84% <0%> (-1.28%)	⬇️
models/user.go	44.3% <0%> (+0.28%)	⬆️

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