package.bbclass: do not split debug pkg when no debugsources.list

package.bbclass: do not split debug pkg when no debugsources.list

We don't have to split the debug pkg if it is null (No debugsources.list
since the find-debuginfo.sh would not generate it for the null pkg).

Serval pkg's debug pkg is null, and there were a few errors: (53 pkgs
had such errors in a core-image-sato build):

log.do_package:sort: open failed: debugsources.list: No such file or directory

[YOCTO #2076]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>