[PATCH] Recipes containing both git and npmsw sources in the SRC_URI fail during fetch from the shrinkwrap. It seems that when the fetcher is fetching from the shrinkwrap, the SRCREV variable has been deleted but it still ends up fetching from the git source resulting in an error because SRCREV is undefined. The root cause of this is that the Fetch class defaults to urls from the SRC_URI when the urls parameter contains an empty list. This patch will ensure that Fetch is not instantiated if the urls list is empty.