classes/create-spdx: handle if IMAGE_LINK_NAME is empty

If IMAGE_LINK_NAME is set to "" (supported in other classes) then skip
creating symlinks for the spdx manifest files.

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>