Add Parameter if source value is emtpy

Hi,

great module, thanks for it!

If a source field's value is empty, the token expression is printed - which is ugly. It may be intended to ignore a value.

It would be great if an empty source field value would be treated as such by not writing anything of it into the target field.
It would be even better if this could be configured.

So if I would like to combine a string of two fields, the current behaviour e.g.:

"Contained things: [emtpy_field_token_printed], some tokenized value"

should become

"Contained things: , some tokenized value"

Then it is completely up to the user to design his strings.