Avoid null payload in mutation

[IvanGoncharov]

Fixes #192

[codecov]

Codecov Report

Merging #268 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #268   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files          19       19           
  Lines         617      618    +1     
  Branches       36       36           
=======================================
+ Hits          614      615    +1     
  Misses          3        3           

Impacted Files	Coverage Δ
src/mutation/__tests__/mutation.js	96.49% <100.00%> (+0.19%)	⬆️
src/mutation/mutation.js	100.00% <100.00%> (ø)

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