Fix angular ticks nre 2012

[beto-rodriguez]

fixes #2012

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[Copilot]

Pull request overview

This PR fixes a NullReferenceException that occurs when the MaxValue of a PieChart with angular ticks changes dynamically. The issue occurred because the Subseparator array in angular ticks is intentionally sparse - it's created with size 5 but only indices 0-3 are initialized, leaving index 4 as null. When the chart scale changes and old tick visuals are removed, the cleanup code attempted to remove all subseparators including the null ones, causing the crash.

Changes:

• Added a null check before calling RemoveGeometryFromPaintTask on subseparators in the cleanup loop
• Added a regression test that reproduces the issue by creating a PieChart with angular ticks and changing its MaxValue

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/LiveChartsCore/VisualElements/BaseAngularTicksVisual.cs	Added null check in cleanup loop to handle intentionally null subseparator array elements
tests/CoreTests/ChartTests/ChartTests.cs	Added regression test that changes MaxValue on a PieChart with angular ticks to verify the fix

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