fix: apply empty arrays as default values but not undefined values

[edwardfoyle]

Issue #, if available:
#5681

fixes an issue that caused some tests to fail in PR #6071 which were reverted in #6179

Description of changes:
Use _.assignWith with a customizer to not assign undefined values instead of using _.merge which would not overwrite empty arrays

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #6445 (00ac44d) into master (4ce4138) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6445      +/-   ##
==========================================
+ Coverage   57.30%   57.34%   +0.03%     
==========================================
  Files         474      474              
  Lines       21669    21671       +2     
  Branches     4090     4091       +1     
==========================================
+ Hits        12418    12427       +9     
+ Misses       8390     8383       -7     
  Partials      861      861              

Impacted Files	Coverage Δ
.../awscloudformation/utils/auth-defaults-appliers.ts	89.65% <100.00%> (+26.69%)	⬆️

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[github-actions]

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