Config Schema: Treat null or undefined as lack of value

[medikoo]

Issue was exposed by CI fail: https://github.com/serverless/serverless/runs/1134654637

(in test we reset websocketApiId with a null value:

serverless/test/integration/websocket.test.js

Line 151 in e1ca63c

apiGateway: { websocketApiId: null },

which seems as totally valid approach)

Workaround to: ajv-validator/ajv#1287

[codecov-commenter]

Codecov Report

Merging #8272 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #8272      +/-   ##
==========================================
+ Coverage   88.08%   88.10%   +0.01%     
==========================================
  Files         248      248              
  Lines        9370     9384      +14     
==========================================
+ Hits         8254     8268      +14     
  Misses       1116     1116              

Impacted Files	Coverage Δ
lib/classes/ConfigSchemaHandler/index.js	89.52% <100.00%> (+1.61%)	⬆️

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