never return null font entry if font not resolved.

src/Skia/Avalonia.Skia/FormattedTextImpl.cs

@@ -38,7 +38,7 @@ namespace Avalonia.Skia
                 IsAntialias = true,
                 LcdRenderText = true,
                 SubpixelText = true,
-                Typeface = entry?.SKTypeface,
+                Typeface = entry.SKTypeface,
                 TextSize = (float)fontSize,
                 TextAlign = textAlignment.ToSKTextAlign()
             };

src/Skia/Avalonia.Skia/SKTypefaceCollection.cs

@@ -31,7 +31,7 @@ namespace Avalonia.Skia
 
             return _fontFamilies.TryGetValue(familyName, out var fontFamily) ?
                 fontFamily.GetOrAdd(key, GetFallback(fontFamily, key)) :
-                null;
+                new TypefaceCollectionEntry(Typeface.Default, SkiaSharp.SKTypeface.Default);
         }
 
         private static TypefaceCollectionEntry GetFallback(IDictionary<FontKey, TypefaceCollectionEntry> fontFamily, FontKey key)