gather: Allow empty result for gather

This only affects Yum backend, and allows to mention packages in comps
groups before they become available. This means a compose can be set up
ahead of all packages being available, and it will simply produce empty
repos.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/gather.py

@@ -950,9 +950,6 @@ class Pungi(PungiBase):
                     if name_arch in prepopulate_packages:
                         self.prepopulate_packages.add(po)
 
-        if not self.po_list:
-            raise RuntimeError("No packages found")
-
         self.logger.info('Finished gathering package objects.')
 
     def gather(self):