Empty default value in --help output for option with multiple=True

[pbeza]

Minimal reproducible example that demonstrates the bug

#!/usr/bin/env python3

import click

@click.command()
@click.option('--multioption', multiple=True, default=None, help='My help', show_default=True)
def info(_multioption):
    pass


if __name__ == '__main__':
    info()

Notice multiple=True which is crucial. If you replace it with multiple=False it will work just fine.

What I get

$ ./test.py --help
Usage: test.py [OPTIONS]

Options:
  --multioption TEXT  My help  [default: ]
  --help              Show this message and exit.  [default: False]

Expected output

$ ./test.py --help
Usage: test.py [OPTIONS]

Options:
  --multioption TEXT  My help
  --help              Show this message and exit.  [default: False]

Environment

Python version

$ python3 --version
Python 3.9.2

Click version

$ python -c "import click; print(click.__version__)"
8.0.1

(I checked that 8.0.2.dev0 is also affected)