'If the metadata file is not JSON encoded return ``None``.'

def test_not_json(self):

    with temporary_mount(self.device.device) as mountpoint:
        metadata_file = mountpoint.descendant(METADATA_RELATIVE_PATH)
        metadata_file.parent().makedirs()
        metadata_file.setContent(random_name(self))
    result = metadata_from_config_drive(config_drive_label=self.label)
    self.assertIs(None, result)
