don't fail on missing check functions

[GantMan]

added a test, too.

[codecov]

Codecov Report

Merging #197 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #197      +/-   ##
=========================================
+ Coverage   82.07%   82.1%   +0.02%     
=========================================
  Files          33      33              
  Lines         636     637       +1     
  Branches      112     113       +1     
=========================================
+ Hits          522     523       +1     
  Misses         78      78              
  Partials       36      36

Impacted Files	Coverage Δ
src/extensions/functions/checkRequirement.ts	79.74% <100%> (+0.25%)	⬆️

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