unified-iso: handle empty arch

Prior to this change, when running pungi-create-unified-iso on a compose
with zero builds present for an arch, unified-iso crashes.

The problem is that unified-iso does not set up the arch's debuginfo
destination directory at all before trying to dump the productmd
treeinfo for that arch's debuginfo. productmd tries to write to the
destination directory that does not exist.

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>

pungi_utils/unified_isos.py

@@ -248,6 +248,7 @@ class UnifiedISO(object):
         for arch, ti in self.treeinfo.iteritems():
             print("Writing treeinfo: {0}".format(arch))
             ti_path = os.path.join(self.temp_dir, "trees", arch, ".treeinfo")
+            makedirs(os.path.dirname(ti_path))
             ti.dump(ti_path)
 
     def discinfo(self):