Hooks: Fix undefined first argument on consecutive action callbacks

[aduth]

Regression introduced in #85
Related: WordPress/gutenberg#5588

This pull request seeks to resolve an issue where all but the first action callbacks will receive undefined first argument. This is caused by an intended simplification of the first argument assignment in the createRunHook.js file. However, this treated actions as the same as filters, thus because most actions do not define a return value, the argument would become unset.

Testing instructions:

Verify unit tests pass:

npm test

[codecov]

Codecov Report

Merging #90 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
+ Coverage   73.45%   73.57%   +0.12%     
==========================================
  Files          35       35              
  Lines         437      439       +2     
  Branches       82       83       +1     
==========================================
+ Hits          321      323       +2     
  Misses         98       98              
  Partials       18       18

Impacted Files	Coverage Δ
packages/hooks/src/createRunHook.js	94.73% <100%> (+0.61%)	⬆️

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