feat(unierr): allow nil errors (close #125)

[GGXXLL]

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[codecov]

Codecov Report

Merging #126 (9b56828) into master (7192cff) will increase coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   76.24%   76.45%   +0.20%     
==========================================
  Files          82       82              
  Lines        3077     3083       +6     
==========================================
+ Hits         2346     2357      +11     
+ Misses        544      539       -5     
  Partials      187      187              

Impacted Files	Coverage Δ
unierr/error.go	55.64% <100.00%> (+3.95%)	⬆️
dtx/sagas/mysqlstore/mysql_store.go	87.34% <0.00%> (+3.79%)	⬆️

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