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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