Fail on missed descr

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/local/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/gram/Documents/gram-lang/corgi/__main__.py", line 4, in <module>
    exit(main(sys.argv[1:]))
  File "/home/gram/Documents/gram-lang/corgi/cli.py", line 144, in main
    return root_parser.handle(argv=argv)
  File "/home/gram/.local/lib/python3.7/site-packages/dephell_argparse/_parser.py", line 225, in handle
    self._print_message(self.format_help())
  File "/home/gram/.local/lib/python3.7/site-packages/dephell_argparse/_parser.py", line 163, in format_help
    self._format_commands(formatter=formatter, command=command)
  File "/home/gram/.local/lib/python3.7/site-packages/dephell_argparse/_parser.py", line 202, in _format_commands
    help=handler.summary,
  File "/home/gram/.local/lib/python3.7/site-packages/dephell_argparse/_cached_property.py", line 18, in __get__
    value = obj.__dict__[self.func.__name__] = self.func(obj)
  File "/home/gram/.local/lib/python3.7/site-packages/dephell_argparse/_handler.py", line 116, in summary
    return self.parser.description.split('\n')[0]
AttributeError: 'NoneType' object has no attribute 'split'

复现 CLI 帮助失败，然后检查 dephell_argparse/_handler.py 中 summary 属性附近的代码，以及 traceback 所显示的格式化路径。完成标准是：在渲染帮助时缺少 parser 描述不再引发 AttributeError。