shield not working without a file

[farfromrefug]

I know it might makes no sense said like this. But it would actually be useful for shield to be usable without file.
The reasoning is for styles like city name where you often show then with a dot (shield image) at low zoom level then without the dot at higher zoom level.
Being able to use none for file would make the style much simpler without the need to use text in one case and shield in the other

[mtehver]

I believe the current behavior copies Mapnik, but adding support for shield-file: none makes sense. Will add it soon.