fix #325 - null simple keys not properly nullified

[Herringway]

This was a real pain to track down.

[codecov]

Codecov Report

Merging #327 (a7c32b3) into master (4196fb8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #327   +/-   ##
=======================================
  Coverage   93.74%   93.74%           
=======================================
  Files          29       29           
  Lines        4571     4573    +2     
=======================================
+ Hits         4285     4287    +2     
  Misses        286      286           

Files	Coverage Δ
source/dyaml/loader.d	94.36% <100.00%> (+0.08%)	⬆️
source/dyaml/scanner.d	99.65% <100.00%> (+<0.01%)	⬆️

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