How to do conditional literal in plugboard?

On my device I prefer the file to be {series}{series_index:0>2s}-{title}. This works when there's a series & sequence. When there's no series I get the hyphen in front of the title. Can I make the hyphen conditional so it only appears when there's a series? If so, can I also make the index conditional so it only appears when the value is greater than 0? There are some 'series' that are non-sequential. Since Calibre doesn't allow a series without an index the indexes in these are all set to 0.

Thanks.