refactor(nb-icon): return null if icon wasn't found

[mmfKupl]

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Short description of what this resolves:

remove icon not found error thrown so that you can dynamically change the icon, and so that after the first installation of the wrong icon, you can change it

[codecov]

Codecov Report

Merging #2466 into master will decrease coverage by 0.04%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #2466      +/-   ##
==========================================
- Coverage   80.46%   80.41%   -0.05%     
==========================================
  Files         244      244              
  Lines        7172     7180       +8     
  Branches      785      787       +2     
==========================================
+ Hits         5771     5774       +3     
- Misses       1168     1171       +3     
- Partials      233      235       +2     

Impacted Files	Coverage Δ
.../framework/theme/components/icon/icon.component.ts	87.93% <33.33%> (-8.15%)	⬇️
.../framework/theme/components/icon/icon-libraries.ts	87.27% <100.00%> (+0.23%)	⬆️