Fix XSLT when no weight system is given

Summing up weights ended only when there was one parameter left. With no
parameters we ended up in an infinite loop. Now we end also in case of
no parameters.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>