when using the %exposed token and empty filters, the token is not replaced

..which makes sense but its not that pretty to get an "export-%exposed.csv" file out of it.. either replace it with an empty string, but im sure everyone uses separators and "export-.csv" wouldnt be nice either..
so we could ask the user to list his separator, trim that, or guess the most used separators.. or what i did in the patch below, use a default string.. like "default", or "no_filters"

patch attached in comment