Review usage of string.Substring (part 1)

Reduced allocations by replacing string.Substring with ReadOnlySpan<char>.Slice

MediaBrowser.MediaEncoding/Subtitles/VttWriter.cs

@@ -47,7 +47,7 @@ namespace MediaBrowser.MediaEncoding.Subtitles
                     text = Regex.Replace(text, @"\\n", " ", RegexOptions.IgnoreCase);
 
                     writer.WriteLine(text);
-                    writer.WriteLine(string.Empty);
+                    writer.WriteLine();
                 }
             }
         }