Fix box/unbox of generic types

[josesimoes]

Description

• Add checks for null objects and perform accordingly.

Motivation and Context

• Handles the specific case where a generic type parameter that is a reference type has a null value, which is exactly what happens when a null is adde to a List and then try to search for it.
• Checks if we're trying to unbox a null reference by verifying the data type is DATATYPE_OBJECT and dereferencing to check for null
  . It throws CLR_E_NULL_REFERENCE if the reference is null, which matches the standard .NET CLR behavior. Also allows normal unboxing to proceed for non-null references.
• Boxing null is legal: object x = (object)null;
• Unboxing null is illegal: int x = (int)null; (throws NullReferenceException).

How Has This Been Tested?

• Tested against Merge generics support in mscorlib CoreLibrary#245
• [build with MDP buildId 57901]
• Related with Support for generics <T> Home#782.

Screenshots

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue with code or algorithm)
• New feature (non-breaking change which adds functionality to code)
• Breaking change (fix or feature that would cause existing functionality to change)
• Config and build (change in the configuration and build system, has no impact on code or features)
• Dev Containers (changes related with Dev Containers, has no impact on code or features)
• Dependencies/declarations (update dependencies or assembly declarations and changes associated, has no impact on code or features)
• Documentation (changes or updates in the documentation, has no impact on code or features)

Checklist

• My code follows the code style of this project (only if there are changes in source code).
• My changes require an update to the documentation (there are changes that require the docs website to be updated).
• I have updated the documentation accordingly (the changes require an update on the docs in this repo).
• I have read the CONTRIBUTING document.
• I have tested everything locally and all new and existing tests passed (only if there are changes in source code).

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[josesimoes]

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