'L{changePassPhrase} doesn\'t modify the key file if C{toString} returns
an empty string.'

def test_changePassphraseEmptyStringError(self):

    filename = self.mktemp()
    FilePath(filename).setContent(privateRSA_openssh)
    def toString(*args, **kwargs):
        return ''
    self.patch(Key, 'toString', toString)
    error = self.assertRaises(SystemExit, changePassPhrase, {'filename': filename, 'newpass': 'newencrypt'})
    if _PY3:
        expected = "Could    not    change    passphrase:    cannot    guess    the    type    of    b''"
    else:
        expected = "Could    not    change    passphrase:    cannot    guess    the    type    of    ''"
    self.assertEqual(expected, str(error))
    self.assertEqual(privateRSA_openssh, FilePath(filename).getContent())
