Fix Unmarshaler call when value is missing

[pelletier]

Fixes #431

[codecov]

Codecov Report

Merging #439 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #439   +/-   ##
=======================================
  Coverage   93.89%   93.90%           
=======================================
  Files          10       10           
  Lines        2441     2443    +2     
=======================================
+ Hits         2292     2294    +2     
  Misses        108      108           
  Partials       41       41           

Impacted Files	Coverage Δ
marshal.go	96.30% <100.00%> (+0.01%)	⬆️

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