insane: only check ${S} exists if we had sources to fetch

insane: only check ${S} exists if we had sources to fetch

Only check that ${S} actually exists if there was something in ${SRC_URI} to
fetch, the argument being that if SRC_URI is empty the the recipe won't be using
${S} at all.

In general recipes that have no sources can remove the unpack task, but
expecting all recipes to do this relatively advanced operation isn't realistic.

Signed-off-by: Ross Burton <ross.burton@intel.com>