Backup of files with unnamed group id fails

[Stephan Müller]

I ran into this issue when I tried to backup files that have group ids which are not resolvable to group names.
The problem is, that when a file does not have a group name, the default is b''. In such a case, the function stn() fails because a bytes object does not have the encode function.

To fix this issue, the line "ti.gname = b''" in path.py must be replaced with "ti.gname = u''".

» ls -lh /tmp/test
total 4,0K
-rw-r--r-- 1 82 82 5 Aug 1 23:09 hallo

» duplicity full --no-encryption /tmp/test onedrive://backup/
Local and Remote metadata are synchronized, no sync needed.
Last full backup date: Thu Aug 1 20:53:32 2019
Traceback (innermost last):
  File "/usr/local/bin/duplicity", line 1711, in <module>
    with_tempdir(main)
  File "/usr/local/bin/duplicity", line 1697, in with_tempdir
    fn()
  File "/usr/local/bin/duplicity", line 1543, in main
    do_backup(action)
  File "/usr/local/bin/duplicity", line 1667, in do_backup
    full_backup(col_stats)
  File "/usr/local/bin/duplicity", line 573, in full_backup
    globals.backend)
  File "/usr/local/bin/duplicity", line 432, in write_multivol
    at_end = gpg.GzipWriteFile(tarblock_iter, tdp.name, globals.volsize)
  File "/usr/local/lib/python3.7/site-packages/duplicity/gpg.py", line 454, in GzipWriteFile
    new_block = next(block_iter)
  File "/usr/local/lib/python3.7/site-packages/duplicity/diffdir.py", line 542, in __next__
    result = self.process(next(self.input_iter)) # pylint: disable=assignment-from-no-return
  File "/usr/local/lib/python3.7/site-packages/duplicity/diffdir.py", line 234, in get_delta_iter
    (new_path, sig_path, sigTarFile))
  File "/usr/local/lib/python3.7/site-packages/duplicity/robust.py", line 41, in check_common_error
    return function(*args)
  File "/usr/local/lib/python3.7/site-packages/duplicity/diffdir.py", line 159, in get_delta_path
    sigTarFile.addfile(ti)
  File "/usr/local/lib/python3.7/tarfile.py", line 1966, in addfile
    buf = tarinfo.tobuf(self.format, self.encoding, self.errors)
  File "/usr/local/lib/python3.7/tarfile.py", line 816, in tobuf
    return self.create_gnu_header(info, encoding, errors)
  File "/usr/local/lib/python3.7/tarfile.py", line 847, in create_gnu_header
    return buf + self._create_header(info, GNU_FORMAT, encoding, errors)
  File "/usr/local/lib/python3.7/tarfile.py", line 937, in _create_header
    stn(info.get("gname", ""), 32, encoding, errors),
  File "/usr/local/lib/python3.7/tarfile.py", line 161, in stn
    s = s.encode(encoding, errors)
 AttributeError: 'bytes' object has no attribute 'encode'

Environment:
duplicity 0.8.02
Python 3.7.3
Ubuntu 19.04