fix: allow matcher and tester is nil

[suzuki-shunsuke]

No description provided.

[codecov-io]

Codecov Report

Merging #16 into master will increase coverage by 0.34%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
+ Coverage   72.69%   73.04%   +0.34%     
==========================================
  Files           5        5              
  Lines         315      319       +4     
==========================================
+ Hits          229      233       +4     
  Misses         65       65              
  Partials       21       21

Impacted Files	Coverage Δ
fagott/tester.go	65.21% <100%> (+0.51%)	⬆️
fagott/matcher.go	88.09% <100%> (+0.29%)	⬆️

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