[release/8.0] Fix deserializing null properties to Protobuf types

[github-actions]

Backport of #52398 to release/8.0

/cc @JamesNK

Fix deserializing null properties to Protobuf types

gRPC JSON transcoding incorrectly doesn't support deserializing null values. The protobuf JSON spec says that parsing null values is supported and the default value should be set.

When parsing JSON-encoded data into a protocol buffer, if a value is missing or if its value is null, it will be interpreted as the corresponding default value.

The fix is to allow deserializing null values and to reset a property to its default value.

Fixes #51742

Customer Impact

Apps are unable to send null values to a gRPC JSON transcoding server. They are either forced to change the client or are stuck if they can't.

There isn't a workaround.

Regression?

• Yes
• No

[If yes, specify the version the behavior has regressed from]

Risk

• High
• Medium
• Low

This is a minor change to the Microsoft.AspNetCore.Grpc.JsonTranscoding package. Nothing else in .NET depends on it.
Null values previously errored. This PR changes their behavior to set a property to its default value instead.

Verification

• Manual (required)
• Automated

After:

Note: null values in JSON payload.

Packaging changes reviewed?

• Yes
• No
• N/A

---

When servicing release/2.1

• Make necessary changes in eng/PatchConfig.props

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[ghost]

Hi @github-actions[bot]. This PR was just approved to be included in the upcoming servicing release. Somebody from the @dotnet/aspnet-build team will get it merged when the branches are open. Until then, please make sure all the CI checks pass and the PR is reviewed.

[JamesNK]

Email approval

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[wtgodbe]

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