fixes NullReferenceException deserializing null sub-objects from json

[beppemarazzi]

PR Type

• Bugfix

What is the current behavior?

Deserializing from json an immutable object containing a reference to another immutable object throws a NullReferenceException if the sub-object is null in json. To reproduce the issue merge only src/Uno.CodeGen.Tests/Given_ImmutableEntity.Newtownsoft.cs leaving current src/Uno.CodeGen/ImmutableGenerator.cs

What is the new behavior?

The object is correctly deserialized with sub-object reference set to null

PR Checklist

• [ ] Tested code with current supported SDKs
• [ ] Docs have been added/updated which fit documentation template. (for bug fixes / features)
• [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
• [ ] Contains NO breaking changes
• [ ] Updated the Release Notes
• [ ] Associated with an issue (GitHub or internal)

Other information

Internal Issue (If applicable):

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:white_check_mark: beppemarazzi
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[carldebilly]

Published as v1.33.0-dev.160 https://www.nuget.org/packages/Uno.CodeGen/1.33.0-dev.160

Uno.CodeGen 1.32.0

This package provides tooling for code generation.