recreate: don't crash on attic archives w/o time_end, fixes #3109

(cherry picked from commit 9d3daebd5fa6a7929fc3a1e556a2b7931c97b354)

src/borg/archive.py

@@ -1697,7 +1697,7 @@ class ArchiveRecreater:
         target.save(comment=comment, additional_metadata={
             # keep some metadata as in original archive:
             'time': archive.metadata.time,
-            'time_end': archive.metadata.time_end,
+            'time_end': archive.metadata.get('time_end') or archive.metadata.time,
             'cmdline': archive.metadata.cmdline,
             # but also remember recreate metadata:
             'recreate_cmdline': sys.argv,