If no files have been staged we want to continue without error instead
of showing a traceback.
Fixes [YOCTO #4056].
(From OE-Core rev: ca36be708e54c0c86535bc8512295c76c48f6cf5)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>