fix: fix nullable issue.

src/Avalonia.Base/Media/Geometry.cs

@@ -214,13 +214,13 @@ namespace Avalonia.Media
             return base.CanConvertFrom(context, sourceType);
         }
 
-        public override object ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object value)
+        public override object? ConvertFrom(ITypeDescriptorContext? context, CultureInfo? culture, object value)
         {
             if (value is null)
             {
                 throw GetConvertFromException(value);
             }
-            string source = value as string;
+            string? source = value as string;
 
             if (source != null)
             {