[JSDSERVER-3149] Customer Portal does not render 'None' for non required custom field (radio button)

Create a custom (radio button) select list field with "none" as default value
Add the field to a Customer Portal form as a non required field
When creating a Service Desk ticket, the field defaults to the first option, rather than showing None

As with standard Jira tickets, the choice "none" should be available to allow user to unselect a choice done by mistake

Jira Service Management Data Center