Add support to process null in attributes

[josesimoes]

Description

• Checking for sentinel value in string ID creating a null string instance.

Motivation and Context

• Add support for attributes with null.
• Following Attribute value now accepts null string metadata-processor#201.

How Has This Been Tested?

• Running MDP unit tests with [DataRow(999, null)].
• Tested against Merge generics support in mscorlib CoreLibrary#245
  [build with MDP buildId 56307]

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).

Summary by CodeRabbit

• Bug Fixes
  • Improved attribute parsing to correctly handle special cases where a string token represents a null value, ensuring more accurate processing of attribute data.

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Walkthrough

The update modifies the CLR_RT_AttributeParser::ReadString method to recognize a special string token value (0xFFFF) as representing a null string. When this value is encountered, the method now creates a null string instance instead of a normal string, before boxing the value as previously.

Changes

File(s)	Change Summary
src/CLR/Core/TypeSystem.cpp	Updated ReadString to handle 0xFFFF token as a null string, creating a null string instance.

Sequence Diagram(s)

sequenceDiagram
    participant Caller
    participant AttributeParser
    participant StringInstance

    Caller->>AttributeParser: ReadString(blob)
    AttributeParser->>AttributeParser: Read token from blob
    alt token == 0xFFFF
        AttributeParser->>StringInstance: Create null string instance
    else token != 0xFFFF
        AttributeParser->>StringInstance: Create string instance from token
    end
    AttributeParser->>AttributeParser: Box string value
    AttributeParser-->>Caller: Return boxed string

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[josesimoes]

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Actionable comments posted: 1

📜 Review details

Configuration used: CodeRabbit UI
Review profile: CHILL
Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 38faac2 and 4870075.

📒 Files selected for processing (1)

• src/CLR/Core/TypeSystem.cpp (1 hunks)

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