Add an empty array element in all multi-checkboxes

so the field key exists and saves as empty, #196
Added a value to the extra, empty added array element

So anyone using the values won't find an empty value which can possibly match with anything and cause a severe security flaw, like for example matching user roles, a user with an empty role can pass as valid.
Resolves #196
Merge pull request #201 from x-team/issue-196

Fix multi-check elements behavior on unchecking all members