Sample app crashes on iOS

[janusw]

When running the sample app (OxyplotMauiSample) on iOS and openening the PanModePage, the app crashes with the following exception:

Exception thrown: 'System.NullReferenceException' in OxyplotMauiSample.dll

	OxyplotMauiSample[928:51189] System.ArgumentNullException: Value cannot be null. (Parameter 'path2')
	   at System.ArgumentNullException.Throw(String paramName)
	   at System.ArgumentNullException.ThrowIfNull(Object argument, String paramName)
	OxyplotMauiSample[928:51189]    at System.IO.Path.Combine(String path1, String path2)
	   at OxyPlot.Maui.Skia.ios.MauiFontLoader.ResolveFileSystemFont(String filename) in oxyplot-maui/Source/OxyPlot.Maui.Skia/Platforms/iOS/MauiFontLoader.cs:line 47
	OxyplotMauiSample[928:51189]    at OxyPlot.Maui.Skia.ios.MauiFontLoader.Load(String fontName) in oxyplot-maui/Source/OxyPlot.Maui.Skia/Platforms/iOS/MauiFontLoader.cs:line 23

This happens both at version 1.1.0 (.NET 9) and on the current develop branch (.NET 10).

[janusw]

Fixed via #27.