Fix license argument check

Earlier I replaced all checks on $License valiue when I should have
replaced all *but this one*.

src/stub

         *)        usage ;;
     esac
     updating && init_from_existing
-    if $MkLicense; then
+    if test -n $License; then
         known_licenses | grep -qxF "$License" \
          || die "unknown license (use -L to get list): $License"
         MkLicense=true