MauiFontLoader.Load: use fontName as fallback if GetFont fails (on iOS)

* and mirror the same behavior on MacCatalyst
* as suggested by Copilot

Author: janusw

Source/OxyPlot.Maui.Skia/Platforms/MacCatalyst/MauiFontLoader.cs

@@ -12,11 +12,11 @@ public Stream Load(string fontName)
         if (fontRegistrar == null)
             fontRegistrar = IPlatformApplication.Current.Services.GetRequiredService<IFontRegistrar>();
 
-        fontName = fontRegistrar.GetFont(fontName);
-        if (File.Exists(fontName))
-        {
+        fontName = fontRegistrar.GetFont(fontName) ?? fontName;
+        if (fontName == null)
+            return null;
+        else if (File.Exists(fontName))
             return File.OpenRead(fontName);
-        }
 
         try
         {

Source/OxyPlot.Maui.Skia/Platforms/iOS/MauiFontLoader.cs

@@ -12,7 +12,7 @@ public Stream Load(string fontName)
         if (fontRegistrar == null)
             fontRegistrar = IPlatformApplication.Current.Services.GetRequiredService<IFontRegistrar>();
 
-        fontName = fontRegistrar.GetFont(fontName);
+        fontName = fontRegistrar.GetFont(fontName) ?? fontName;
         if (fontName == null)
             return null;
         else if (File.Exists(fontName))