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
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'
```

Reproduce the CLI help failure, then inspect dephell_argparse/_handler.py around the summary property and the formatting path shown in the traceback. Done means a missing parser description no longer raises AttributeError when help is rendered.

Written by the indexing model from the issue text.