allow multiple=True with flags

[davidism]

#2246 reported that multiple=True didn't work with flags since default=() was also required. I can't remember the original conversation around that, but instead of setting the default, #2248 made multiple=True with flags an error. Comments on the PR, as well as #2292 and #2295 reported that this broke valid uses.

This removes the error and instead sets default=() if multiple=True for flags. This works if the option is a flag due to a secondary token (-y/-n), is_flag=True, or flag_value=value.

fixes #2246
fixes #2292
fixes #2295

[davidism]

OK, I did find #2247, which is essentially the same as this PR, and we closed that in favor of the error because it seemed like count=True should be more appropriate than multiple=True, is_flag=True. But that was because there were no test cases demonstrating uses of that pattern, and now people posted a bunch of examples, so I think this is the correct fix.