fix(#255): handle falsy children values

[doctyper]

Fixes #255

[codecov]

Codecov Report

Merging #256 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #256      +/-   ##
==========================================
+ Coverage   98.54%   98.55%   +0.01%     
==========================================
  Files           3        3              
  Lines         275      277       +2     
==========================================
+ Hits          271      273       +2     
  Misses          4        4

Impacted Files	Coverage Δ
src/Helmet.js	100% <100%> (ø)	⬆️

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