`required=True` does not work for flags

[rockwellw]

When using a flag option, setting required=True has no effect unless I also include default=None. I would expect there to be no default if the flag is required.

Steps to reproduce:

import click

@click.command()
@click.option('--foo/--no_foo', is_flag=True, required=True)
def bar(foo):
    pass

if __name__ == '__main__':
    bar()

python test.py runs without raising an error.

I would expect python test.py to raise:

Usage: test.py [OPTIONS]
Try 'test.py --help' for help.

Error: Missing option '--foo'.

Environment:

• Python version: 3.9.5
• Click version: 8.0.1