HAL-1061 Add ability to ignore an empty item in ComboItem

[spyrkob]

https://issues.jboss.org/browse/HAL-1061

[hpehl]

Thanks for the PR. I would suggest to turn the setter into a second constructor signature to specify whether this combobox allows empty values or not. IMO it's something which should be set at construction time but cannot be changed later on.

public ComboBoxItem(String name, String title) {
    this(name, title, true);
}

public ComboBoxItem(String name, String title, boolean allowEmpty) {
    super(name, title);
    this.allowEmpty = allowEmpty;
    ...
}

I'm going to merge the remaining HAL PRs once this PR is merged.

[spyrkob]

I added the constructor, but didn't change the field name. "allowEmpty" seems a bit misleading - if there's an empty item in the values collection, this will not prevent it from being displayed. Let me know if that's OK.