Even if gather phase is skipped, it will still generate a rpms.json
metadata file. It will have no payload, but the header structure is
there.
Createrepo phase had to be updated to ignore variants that have no RPMs
listed in metadata.
There is one more fix for writing treeinfo files: if repomd.xml files
are not generated, it will no longer crash.
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>