CronExpression.GetFriendlyDescription() should not throw exception when expression is empty

Author: muratcakir

src/Libraries/SmartStore.Services/Tasks/CronExpression.cs

@@ -59,14 +59,16 @@ public static string GetFriendlyDescription(string expression)
 				Use24HourTimeFormat = Thread.CurrentThread.CurrentUICulture.DateTimeFormat.AMDesignator.IsEmpty()
 			};
 
-			try
-			{
-				return ExpressionDescriptor.GetDescription(expression, options);
-			}
-			catch
+			if (expression.HasValue())
 			{
-				return null;
+				try
+				{
+					return ExpressionDescriptor.GetDescription(expression, options);
+				}
+				catch { }
 			}
+
+			return "?";
 		}
 
 	}