Fix empty CSS declarations overriding presentation attributes

[wieslawsoltes]

Pull Request Summary

Summary

Fixes the regression reported in #231 where an SVG presentation attribute such as fill="rgb(0,0,255)" was incorrectly overridden by an invalid empty CSS declaration such as style="fill:;".

Browsers drop declarations with no value and continue applying later valid declarations in the same style block. The Svg.Skia compatibility CSS pipeline was instead forwarding empty declaration values into SvgElement.AddStyle, which allowed the empty inline or stylesheet value to win over a valid presentation attribute and made the fill resolve as black.

This change makes the compatibility pipeline ignore empty or whitespace-only declaration values in both places where CSS declarations are converted into SVG styles:

• Inline style="..." attributes handled by SvgInlineStyleAttributeParser.
• Aggregated stylesheet rules handled by SvgCssCompatibilityProcessor.

Valid declarations following an invalid empty declaration are still applied, matching browser behavior. For example, style="fill:;stroke:white" now preserves the element's presentation fill and still applies the inline stroke.

Changes

• Added a guard in the fast inline style parser so empty values parsed from style="fill:;" are skipped instead of added to the element style map.
• Added the same guard in the ExCSS fallback path for inline styles, so malformed inline style text that falls back to ExCSS has the same behavior.
• Added the same guard when stylesheet declarations from <style> elements are materialized for selector application.
• Added regression coverage for:
  • The exact blue/red/blue inline-style scenario from issue Regression with SVG attribute (fill specified, style empty) #231.
  • Inline styles that force the ExCSS fallback path.
  • <style> stylesheet rules containing empty declarations.
  • Preservation of later valid declarations such as stroke after an invalid fill:.

Validation

• dotnet test tests/Svg.Skia.UnitTests/Svg.Skia.UnitTests.csproj -f net10.0 -c Release --no-restore --filter "FullyQualifiedName~SvgDocumentCompatibilityLoaderTests" passed with 37 tests.
• Prior broader verification on this branch state passed:
  • dotnet build Svg.Skia.slnx -c Release --no-restore
  • dotnet test Svg.Skia.slnx -c Release --no-restore

Notes

dotnet format was attempted during validation, but the formatter host did not return in this worktree for either solution-scoped or project-scoped invocations. The touched source and test changes are minimal and follow the surrounding style.

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