Fix typeface exception when created with empty font family name (#19654)

* add unit test case to check typeface creation with empty font family.

* fallback to default font family when the font family name is empty.

---------

Co-authored-by: jay.mao <jay.mao@ringcentral.com>

src/Avalonia.Base/Media/Typeface.cs

@@ -48,7 +48,8 @@ namespace Avalonia.Media
             FontStyle style = FontStyle.Normal,
             FontWeight weight = FontWeight.Normal,
             FontStretch stretch = FontStretch.Normal)
-            : this(fontFamilyName == null ? FontFamily.Default : new FontFamily(fontFamilyName), style, weight, stretch)
+            : this(string.IsNullOrEmpty(fontFamilyName) ? FontFamily.Default : new FontFamily(fontFamilyName),
+                  style, weight, stretch)
         {
         }
 

tests/Avalonia.Skia.UnitTests/Media/FontManagerTests.cs

@@ -427,5 +427,19 @@ namespace Avalonia.Skia.UnitTests.Media
                 }
             }
         }
+
+        [Fact]
+        public void Should_Fallback_When_Font_Family_Is_Empty()
+        {
+            using (UnitTestApplication.Start(
+                TestServices.MockPlatformRenderInterface.With(fontManagerImpl: new FontManagerImpl())))
+            {
+                using (AvaloniaLocator.EnterScope())
+                {
+                    var typeface = new Typeface(string.Empty);
+                    Assert.NotNull(typeface.FontFamily);
+                }
+            }
+        }
     }
 }