Fix nil pointer deref in distribution tax/rewward validation helpers

[alessio]

Before checking Dec's value, one has to ensure the internal big.Int
pointer is not nil first.

Closes: #5621

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[alexanderbez]

Can you allude to when an Int or Dec would be Nil? This should never be the case.

[alexanderbez]

Please reference the issue in the PR body @alessio

[alexanderbez]

ACK

[alessio]

@alexanderbez @jackzampolin please have another look

[alexanderbez]

@alessio please address the scope linting errors in the tests.

[alexanderbez]

ACK

[codecov]

Codecov Report

Merging #5620 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5620      +/-   ##
==========================================
+ Coverage   44.63%   44.64%   +<.01%     
==========================================
  Files         324      324              
  Lines       24707    24707              
==========================================
+ Hits        11029    11031       +2     
+ Misses      12618    12616       -2     
  Partials     1060     1060

Impacted Files	Coverage Δ
x/mock/app.go	66.46% <0%> (+1.21%)	⬆️