fix(event-sources): handle Cognito claimsOverrideDetails set to null

[michaelbrewer]

Issue #, if available:

• PreTokenGenerationTriggerEvent => claims_to_suppress : NoneType' object does not support item assignment #870

Description of changes:

self._data["response"].setdefault("claimsOverrideDetails", {}) does not work, if claimsOverrideDetails was set to null in the json event (or None in the parsed Python)

Checklist

• Meet tenets criteria
• Update tests
• Update docs
• PR title follows conventional commit semantics

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[codecov-commenter]

Codecov Report

Merging #878 (d7f357c) into develop (0348bd6) will decrease coverage by 0.01%.
The diff coverage is 75.00%.

@@             Coverage Diff             @@
##           develop     #878      +/-   ##
===========================================
- Coverage    99.90%   99.88%   -0.02%     
===========================================
  Files          118      118              
  Lines         5126     5127       +1     
  Branches       571      572       +1     
===========================================
  Hits          5121     5121              
  Misses           2        2              
- Partials         3        4       +1     

Impacted Files	Coverage Δ
.../utilities/data_classes/cognito_user_pool_event.py	99.73% <50.00%> (-0.27%)	⬇️
aws_lambda_powertools/metrics/metric.py	100.00% <100.00%> (ø)

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