Fix behavior when setting options_metavar parameter as empty

[yo-main]

Hi,

I have encountered an issue when using a Command object without any options.
As I wanted to prevent the '[OPTION]' from appearing in the Usage line, I tried to set the options_metavar parameter to None but got an exception.

I also tried to set it as an empty string rather than None, there's no exception then but the behavior isn't perfect neither as it prints a double space in the Usage row (the double space is between the filename and the argument)

Here's a code snippet to reproduce the issue with None

@click.command("foo", options_metavar=None)
@click.argument("bar")
def cli():
    pass

Expected Behavior

Running the above, I expected to see

$ python foobar.py --help
Usage: foobar.py BAR

Options:
  --help  Show this message and exit.

Actual Behavior

An error is raised as the formatter was expecting a string instead of a None.

Traceback (most recent call last):
  File "foobar.py", line 8, in <module>
    cli()
  File "/home/romain/projects/apps/myprss/.venv/lib/python3.7/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/home/romain/projects/apps/myprss/.venv/lib/python3.7/site-packages/click/core.py", line 781, in main
    with self.make_context(prog_name, args, **extra) as ctx:
  File "/home/romain/projects/apps/myprss/.venv/lib/python3.7/site-packages/click/core.py", line 700, in make_context
    self.parse_args(ctx, args)
  File "/home/romain/projects/apps/myprss/.venv/lib/python3.7/site-packages/click/core.py", line 1048, in parse_args
    value, args = param.handle_parse_result(ctx, opts, args)
  File "/home/romain/projects/apps/myprss/.venv/lib/python3.7/site-packages/click/core.py", line 1630, in handle_parse_result
    value = invoke_param_callback(self.callback, ctx, self, value)
  File "/home/romain/projects/apps/myprss/.venv/lib/python3.7/site-packages/click/core.py", line 123, in invoke_param_callback
    return callback(ctx, param, value)
  File "/home/romain/projects/apps/myprss/.venv/lib/python3.7/site-packages/click/core.py", line 950, in show_help
    echo(ctx.get_help(), color=ctx.color)
  File "/home/romain/projects/apps/myprss/.venv/lib/python3.7/site-packages/click/core.py", line 570, in get_help
    return self.command.get_help(self)
  File "/home/romain/projects/apps/myprss/.venv/lib/python3.7/site-packages/click/core.py", line 975, in get_help
    self.format_help(ctx, formatter)
  File "/home/romain/projects/apps/myprss/.venv/lib/python3.7/site-packages/click/core.py", line 1001, in format_help
    self.format_usage(ctx, formatter)
  File "/home/romain/projects/apps/myprss/.venv/lib/python3.7/site-packages/click/core.py", line 923, in format_usage
    formatter.write_usage(ctx.command_path, " ".join(pieces))
TypeError: sequence item 0: expected str instance, NoneType found

Environment

• Python version: 3.7.4
• Click version: 7.1.2

[yo-main]

Hi

This PR looks ok to you ?
Or is there anything missing ?

Let me know otherwise so I can close it

Thanks !