Add a single argument Parse function for FontFamily.

src/Avalonia.Visuals/Media/FontFamily.cs

@@ -133,6 +133,16 @@ namespace Avalonia.Media
             }
         }
 
+        /// <summary>
+        /// Parses a <see cref="T:Avalonia.Media.FontFamily"/> string.
+        /// </summary>
+        /// <param name="s">The <see cref="T:Avalonia.Media.FontFamily"/> string.</param>
+        /// <returns></returns>
+        /// <exception cref="ArgumentException">
+        /// Specified family is not supported.
+        /// </exception>
+        public static FontFamily Parse(string s) => Parse(s, null);
+
         /// <summary>
         /// Parses a <see cref="T:Avalonia.Media.FontFamily"/> string.
         /// </summary>
@@ -142,7 +152,7 @@ namespace Avalonia.Media
         /// <exception cref="ArgumentException">
         /// Specified family is not supported.
         /// </exception>
-        public static FontFamily Parse(string s, Uri baseUri = null)
+        public static FontFamily Parse(string s, Uri baseUri)
         {
             if (string.IsNullOrEmpty(s))
             {