feat(graphql-key-transformer): only modify GSI sort key when present

[edwardfoyle]

Issue #, if available:
#6634
Description of changes:
Only sets the composite sort key in the VTL resolver if all fields in the key are preset in the mutation.

Also adds unit and e2e tests for this case.

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 #6742 (4b7225c) into master (bda37b4) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6742      +/-   ##
==========================================
+ Coverage   56.80%   56.82%   +0.02%     
==========================================
  Files         486      486              
  Lines       22033    22034       +1     
  Branches     4402     4403       +1     
==========================================
+ Hits        12515    12521       +6     
+ Misses       8641     8636       -5     
  Partials      877      877              

Impacted Files	Coverage Δ
...nction-runtime-provider/src/utils/legacyPackage.ts	0.00% <ø> (ø)
...ages/graphql-key-transformer/src/KeyTransformer.ts	92.93% <100.00%> (+1.15%)	⬆️

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[attilah]

LGTM, good job! 🥇 the tests!

[github-actions]

This pull request has been automatically locked since there hasn't been any recent activity after it was closed. Please open a new issue for related bugs.

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