Added guards in TryGetGlyphTypeface so the $Default placeholder stops… (#19891)

* Added guards in TryGetGlyphTypeface so the $Default placeholder stops recursing once the concrete default family cannot be resolved

* Throw if $Default is used as the default family name

* Stripped the redundant $Default guard/continue blocks

---------

Co-authored-by: Julien Lebosquain <julien@lebosquain.net>

src/Avalonia.Base/Media/FontManager.cs

@@ -163,7 +163,7 @@ namespace Avalonia.Media
             }
 
             //Nothing was found so use the default
-            return TryGetGlyphTypeface(new Typeface(FontFamily.DefaultFontFamilyName, typeface.Style, typeface.Weight, typeface.Stretch), out glyphTypeface);
+            return TryGetGlyphTypeface(new Typeface(DefaultFontFamily, typeface.Style, typeface.Weight, typeface.Stretch), out glyphTypeface);
 
             FontFamily GetMappedFontFamily(FontFamily fontFamily)
             {
@@ -380,6 +380,12 @@ namespace Avalonia.Media
                     "Default font family name can't be null or empty.");
             }
 
+            if (defaultFontFamilyName == FontFamily.DefaultFontFamilyName)
+            {
+                throw new InvalidOperationException(
+                    $"'{FontFamily.DefaultFontFamilyName}' is a placeholder and cannot be used as the default font family name. Provide a concrete font family name via {nameof(FontManagerOptions)} or the platform implementation.");
+            }
+
             return defaultFontFamilyName;
         }