fix: null deserialization of stageVariables for sam local

[nmoutschen]

Issue #, if available: #365

Description of changes:

Add support for deserializing API Gateway v2 payloads when stageVariables is set to null.

AWS SAM local sends a payload that contains a null value for stageVariables, which causes the deserialization into a lambda_http::request::LambdaRequest to fail. This makes this field deserializable using the nullable_default function to handle that edge case.

Open question: should we also make this change to all fields that deserializes into a StrMap?

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[coltonweaver]

Open question: should we also make this change to all fields that deserializes into a StrMap?

In favor of your robustness comment in the issue related to this, I think it makes sense to do this. Overall the change looks good. Do you want to include this in this PR? I'll approve but feel free to push up the change to do that.

[nmoutschen]

In favor of your robustness comment in the issue related to this, I think it makes sense to do this. Overall the change looks good. Do you want to include this in this PR? I'll approve but feel free to push up the change to do that.

Will do - better to do it in one go. I'll let you review before merging to get another pair of eyes. :)

[coltonweaver]

Looks good, thanks!