license.bbclass: skip license checking if the package contains no file

license.bbclass: skip license checking if the package contains no file

If the package doesn't contain any file, then the license isn't relevant
as far as the final image is concerned. So we skip the license checking
in license_create_manifest if such case.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>