If you set it to @n6.2 then it will always have a value ( even if 0) so a placeholder cannot be displayed.
If you set it to @n6.2B then at least there's the possibility of a placeholder - but since 0 is a legal value, you actually would want that to be displayed.
So I'm not really sure in what context the placeholder would be displayed in this case? Perhaps changing it to a string would be the best approach? Then blank is blank, and a number, even 0, is not blank.
cheers
Bruce