Refactore more scope tests to use isNullOrUndefined(scope)

[danielgranat]

After allowing 0 scope, I found more places in the code that I neglected.

[codecov]

Codecov Report

Merging #857 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #857      +/-   ##
==========================================
+ Coverage   95.09%   95.09%   +<.01%     
==========================================
  Files          47       47              
  Lines        1264     1265       +1     
  Branches      404      405       +1     
==========================================
+ Hits         1202     1203       +1     
  Misses         52       52              
  Partials       10       10

Impacted Files	Coverage Δ
src/core/validator.js	99.28% <100%> (ø)	⬆️
src/core/field.js	100% <100%> (ø)	⬆️
src/core/generator.js	100% <100%> (ø)	⬆️
src/core/errorBag.js	100% <100%> (ø)	⬆️
src/core/utils.js	99.31% <100%> (ø)	⬆️

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[logaretm]

Thanks for the follow up!