Fix encoding span polyfills for empty spans

[thomhurst]

Summary

• short-circuit stateless Encoding span overloads for empty input
• use non-null stack storage for empty destinations and stateful Encoder.Convert
• add empty-source and empty-destination regression coverage
• regenerate Split sources, assembly-size data, and README snippets

Root cause

Pinning an empty span produces a null pointer. The pointer-based encoding overloads reject null pointers even when the corresponding length is zero, unlike the native span overloads.

Validation

• dotnet run --project src/ApiBuilderTests/ApiBuilderTests.csproj --configuration Debug — 71 passed
• dotnet build src/Polyfill.slnx --configuration Release — succeeded, 0 errors
• pwsh ./src/run-tests.ps1 — full project/framework matrix passed

Closes #569

[SimonCropp]

nice. will deploy now