By empty do you mean the None option? If so you'll need to have the field required either at creation time using the field configuration scheme or with a plugin (I use JIRA suite utilities) if it is shown in a transition screen.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.