fix(converters): allow nullable return type in Convert method

[realybin]

Updated the Convert method in the DivisionMathConverter class to return a nullable object type (object?) instead of a non-nullable object. This change allows the method to explicitly return null as a valid value.

[github-actions]

Summary - Code Coverage

Summary

Generated on:	1/22/2025 - 5:37:53 PM
Coverage date:	1/22/2025 - 5:37:50 PM
Parser:	Cobertura
Assemblies:	1
Classes:	18
Files:	27
Line coverage:	27.9% (114 of 408)
Covered lines:	114
Uncovered lines:	294
Coverable lines:	408
Total lines:	2034
Branch coverage:	20.2% (15 of 74)
Covered branches:	15
Total branches:	74
Method coverage:	Feature is only available for sponsors
Tag:	111_12913857055

Coverage

SketchNow - 27.9%

Name	Line	Branch
SketchNow	27.9%	20.2%
SketchNow.App	0%	0%
SketchNow.Controls.CustomInkCanvas	0%
SketchNow.Controls.IconWithText	0%	0%
SketchNow.Converters.ColorToBrushConverter	0%	0%
SketchNow.Converters.DivisionMathConverter	0%	0%
SketchNow.Converters.EnumToIndexConverter	0%	0%
SketchNow.Input.StylusPlugIns.CustomStylusPlugin	0%
SketchNow.Models.CanvasPage	32.3%	25%
SketchNow.Models.CanvasPages	35.2%
SketchNow.Models.CursorNotFoundException	0%
SketchNow.Models.CustomCursors	100%	50%
SketchNow.Properties.Resource	0%	0%
SketchNow.Properties.Settings	45.4%
SketchNow.ViewModels.MainWindowViewModel	62%	40%
SketchNow.Views.MainWindow	0%	0%
SketchNow.Views.PageNavigationView	0%	0%
SketchNow.Views.SettingsView	0%	0%
SketchNow.Views.ToolBarView	0%	0%