Handle null values during deserialization of primitive types

[Havret]

No description provided.

[Copilot]

Pull Request Overview

This PR adds null handling support for primitive protobuf field deserialization. When JSON properties are explicitly set to null, the deserializer now treats them as default values rather than throwing an error.

Key Changes:

• Added null token check in FieldConverter.Read() to skip processing when null values are encountered
• Added comprehensive test coverage for null value handling across all primitive protobuf types

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Protobuf.System.Text.Json/InternalConverters/FieldConverter.cs	Added null token check to return early and use default values when converters don't handle nulls
test/Protobuf.System.Text.Json.Tests/SimpleMessageTests.cs	Added test case verifying that null values deserialize to default values for all primitive types

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[Copilot]

Extra trailing space after closing brace should be removed.

Suggested change

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