fix(support): call toFixed with argument if argument is 0

[dated]

Summary

According to https://mikemcl.github.io/bignumber.js/#toFix num.toFixed() !== num.toFixed(0).

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged

[codecov]

Codecov Report

Merging #640 into master will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #640      +/-   ##
==========================================
+ Coverage   49.52%   49.66%   +0.13%     
==========================================
  Files          53       53              
  Lines         739      741       +2     
  Branches       81       81              
==========================================
+ Hits          366      368       +2     
  Misses        359      359              
  Partials       14       14              

Flag	Coverage Δ
#unit	49.66% <100.00%> (+0.13%)	⬆️

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Impacted Files	Coverage Δ
packages/platform-sdk-support/src/bignumber.ts	100.00% <100.00%> (ø)

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