When combining an SPDX document, the package list might be empty (e.g.
a baremetal image). Handle this case instead of erroring out
(From OE-Core rev: baf4e360f6e65a5e9aff2def69d2a720f38f92b2)
Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>