'None of the above' option does not work

Problem/Motivation

When selecting 'None of the above' option for webform quiz checkboxes upon saving the element validation kicks in and asks to select a correct answer

Steps to reproduce

• Select a webform quiz element
• Add some answers with no answers
• Select the none of the above option
• Save element

Proposed resolution

Upon selecting None of the above this should be the correct answer and not validate the answers for a correct answer selection (as none of the above are correct)