Empty Bitbucket repositories raise "AssertionError: file too short to contain pack"

Sentry Issue: SWH-LOADER-GIT-1G4

AssertionError: file too short to contain pack
(2 additional frame(s) were not displayed)
...
  File "swh/loader/git/loader.py", line 395, in get_contents
    for raw_obj in self.iter_objects(b"blob"):
  File "swh/loader/git/loader.py", line 385, in iter_objects
    PackData.from_file(self.pack_buffer, self.pack_size)
  File "dulwich/pack.py", line 1186, in from_file
    return cls(str(file), file=file, size=size)
  File "dulwich/pack.py", line 1171, in __init__
    (version, self._num_objects) = read_pack_header(self._file.read)
  File "dulwich/pack.py", line 805, in read_pack_header
    raise AssertionError("file too short to contain pack")

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