Handle blank lines when classifying reference spans

[dibarbet]

Summary

• handle reference spans reported on lines with no non-whitespace content
• use the source line start as the widened span fallback for blank lines
• add focused regression coverage for Request textDocument/documentSymbol failed. Message: Value cannot be null. vscode-csharp#8354

Test plan

• dotnet test .\src\Features\Test\Microsoft.CodeAnalysis.Features.UnitTests.csproj --filter "FullyQualifiedName=Microsoft.CodeAnalysis.UnitTests.Classification.ClassifiedSpansAndHighlightSpanFactoryTests.ClassifyAsync_ReferenceOnBlankLine" --no-restore --verbosity minimal
• dotnet build .\src\Features\Core\Portable\Microsoft.CodeAnalysis.Features.csproj --no-restore --verbosity minimal

Fixes dotnet/vscode-csharp#9482

Microsoft Reviewers: Open in CodeFlow

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Pull request overview

This PR updates Roslyn’s reference-span classification logic to safely handle cases where a reported reference span lands on a line with no non-whitespace content (e.g., blank/whitespace-only lines), preventing a crash while producing a reasonable fallback span.

Changes:

• Update GetLineSpanForReference to handle GetFirstNonWhitespacePosition() returning null by falling back to sourceLine.Start.
• Add a focused regression unit test that exercises classification when the reference is on a blank first line.

Show a summary per file

File	Description
src/Features/Core/Portable/ClassifiedSpansAndHighlightSpanFactory.cs	Avoids null dereference when widening a reference span on blank/whitespace-only lines by falling back to the line start.
src/Features/Test/Classification/ClassifiedSpansAndHighlightSpanFactoryTests.cs	Adds a regression test ensuring classification on a blank line returns empty classified spans and a default highlight span (and does not throw).

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite