Preserve Color.Empty when serializing

[AArnott]

Why

The built-in System.Drawing.Color converter loses Color.Empty identity because it only stores ARGB, causing the empty color to deserialize as transparent black.

What changed

Encode Color.Empty as the MessagePack boolean false while retaining ARGB integer encoding for non-empty colors. Deserialization accepts false and existing integer payloads, and rejects true. The schema and docs describe the integer-or-false representation. Tests cover both Color.Empty and transparent black.

Validation

• Focused TUnit tests passed on .NET 8 and .NET 10.
• DocFX build passed.

Closes #1102

[Copilot]

Copilot review overview

🟡 Changes recommended

Nullable empty colors lose their identity, and nullable-color array schemas reject valid null elements.

Review effort: Balanced
Findings: 1 · 1

Open (2)

• Color.Empty encoding loses nullable presence · New
• Nullable color array schema rejects valid null elements · New

What changed in this PR

Updates the built-in color converter to distinguish Color.Empty from transparent black, addressing #1102.

Changes:

• Encodes empty colors as nil while retaining ARGB integers for non-empty colors.
• Updates the schema and documentation.
• Adds round-trip tests for empty colors and transparent black.

File	Description
test/​Nerdbank.MessagePack.TUnit/​BuiltInConverterTests.cs	Tests color identity and encoding.
src/​Nerdbank.MessagePack/​Converters/​BclConverters.cs	Adds nil handling and updates the color schema.
docfx/​docs/​built-in-converters.md	Documents the nil-or-integer representation.

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[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation preserves nullable semantics and integer compatibility, with targeted regression coverage and no blocking issues.

Review effort: Balanced
Findings: None

Resolved since last review (2)

• Color.Empty encoding loses nullable presence
• Nullable color array schema rejects valid null elements

[Copilot]

Copilot review overview

🟢 Approval recommended

The encoding, nullable handling, schemas, and regression coverage are consistent, with no blocking issues identified.

Review effort: Balanced
Findings: None

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.83%. Comparing base (7ea3507) to head (289fce6).

Files with missing lines	Patch %	Lines
...c/Nerdbank.MessagePack/Converters/BclConverters.cs	84.61%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             v1.4    #1103      +/-   ##
==========================================
+ Coverage   82.80%   82.83%   +0.02%     
==========================================
  Files         259      259              
  Lines       21296    21327      +31     
  Branches     2810     2814       +4     
==========================================
+ Hits        17634    17666      +32     
+ Misses       2762     2760       -2     
- Partials      900      901       +1     

Flag	Coverage Δ
Linux	82.30% <94.59%> (-0.09%)	⬇️
Windows	82.55% <94.59%> (+0.04%)	⬆️
macOS	82.29% <94.59%> (+0.05%)	⬆️

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