package.bbclass: do_split_packages should always return something.
So far, when do_split_packages is passed a non-existing folder, it will return nothing.
While building Mesa with PACKAGECONFIG="egl gles" for qemux86, do_package threw a nice exception on a line reading "pkgs += do_split_packages":
"Exception: TypeError: 'NoneType' object is not iterable"
I think a function should always return something, and in this specific case, returning an empty list should be right.
This patch simply fixes the return statement to do just that.
