Cannot deselect 'Send group email'

If you check the 'Send Group Email' box in a workflow:

If you uncheck the box, and save, emails will still be sent via group email.

It appears that this is caused because this line checks if the value is set, but does not check the value itself, so in theory an empty value would return true.

https://github.com/humanmade/Workflows/blob/cac43d591353447e06a89222c8b6f013a186734b/lib/destinations/email.php#L71