When the result of a Virutal Tags function is empty don't add any prefix or suffix.

Author: maforget

cYo.Common/Text/ExtendedStringFormater.cs

@@ -71,8 +71,8 @@ private static string Format(string format, Func<string, object> getValue, out b
                             bool success3;
                             string value3 = Format(GetPart(format, ref i, '$', '>', ['<', '>', '$']), getValue, out success3, true);
                             string result = ParseFunction(value3, out success3);
-                            stringBuilder.Append(result);
-                            success &= success3;
+							stringBuilder.Append(result);
+                            success &= success3 && !string.IsNullOrEmpty(result);
                             break;
                         }
                      case '{':