recipetool: create: only add npmsw url if required

Before adding a npmsw fetcher to a recipe we
should first check if the generated shrinkwrap file
contains dependencies.

Signed-off-by: Kamel Bouhara <kamel.bouhara@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit ef153ad3)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>