Empty example table entries with converter to None documentation

[jsa34]

Add documentation for parser converter for empty strings in example tables.

Resolves #551

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.07%. Comparing base (d527a67) to head (e181ced).
Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #747   +/-   ##
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  Coverage   96.07%   96.07%           
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  Files          55       55           
  Lines        2244     2244           
  Branches      246      246           
=======================================
  Hits         2156     2156           
  Misses         53       53           
  Partials       35       35           

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[jsa34]

@youtux Could you review and see if this is clear/correct/helpful?

[youtux]

do we really need a test for this? isn't this tested somewhere else already? Maybe not this very example, but the fact that converters... basically work.

[jsa34]

Good point. I'll just add to the docs