Fix RestJson serialization of sparse null map values

[AlexDaines]

Description

Modified the JSON marshaller generation for sparse maps to properly handle null values in accordance with the REST JSON protocol specification.

Motivation and Context

Fixes RestJsonSerializesSparseNullMapValues protocol test. The marshaller was not correctly handling null values in sparse maps, causing an InvalidOperationException when attempting to serialize null values.

Testing

Dry-run succeeded @ Feb 27, 2025, 17:59:48.041 (UTC-05:00)

All tests pass, including RestJsonSerializesSparseNullMapValues. The fix ensures proper serialization of null values for:

• Nullable boolean values (bool?)
• Nullable number values (int?)
• String values
• Struct values

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license

[dscpinheiro]

I'm not of the reviewers, but I just merged 3 PRs to v4-development. I recommend doing another dry-run to make sure your changes still pass.

[peterrsongg]

@boblodgett I could be convinced either way. Should we also include the logic for sparse lists even though we don't have a test for this? There are tests right now that we are skipping for example: https://github.com/aws/aws-sdk-net/blob/main/generator/ProtocolTestsGenerator/smithy-dotnet-codegen/src/main/java/software/amazon/smithy/dotnet/codegen/customizations/ProtocolTestCustomizations.java#L36
which target SparseStringList etc. Maybe just backlog it so we are aware

[boblodgett]

I am ok with backlog item for it. Nothing uses it right now anyway and C2J doesn't support sparse.

[peterrsongg]

Looks good, but we have to remember not to overwrite the additions you made in the normal.json file somehow b/c in the next copy/paste we do from the artifact it will get overwritten. Can we add an additional file like:

rest-json-protocol-partial.json

which has just your additions in it?