Ensure to preserve "undefined" values properties when normalizing for schema

[medikoo]

Fixes issue introduced with #8319 where properties with undefined value were converted to null

Closes: #8370

[codecov-io]

Codecov Report

Merging #8374 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #8374   +/-   ##
=======================================
  Coverage   88.03%   88.03%           
=======================================
  Files         249      249           
  Lines        9267     9268    +1     
=======================================
+ Hits         8158     8159    +1     
  Misses       1109     1109           

Impacted Files	Coverage Δ
lib/classes/ConfigSchemaHandler/index.js	90.17% <100.00%> (+0.08%)	⬆️

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