create-spdx: Protect against None from LICENSE_PATH

If LICENSE_PATH is not set, then the split() will fail on a NoneType.

Signed-off-by: Saul Wold <saul.wold@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit d6260dec)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>