Cleanup genesis state validation + Genutil

[alexanderbez]

Closes: #5086

• Ensure gentxs default genesis state is not nil
• Cleanup and add context to ValidateGenesis methods.

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• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
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[codecov]

Codecov Report

Merging #5499 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5499      +/-   ##
==========================================
+ Coverage   54.52%   54.53%   +0.01%     
==========================================
  Files         313      313              
  Lines       18877    18877              
==========================================
+ Hits        10293    10295       +2     
+ Misses       7800     7798       -2     
  Partials      784      784

Impacted Files	Coverage Δ
x/mock/app.go	65.54% <0%> (+1.35%)	⬆️