It should be possible to allow an empty selection even when force selection is on.

[ianjosephwilson]

When is-editable = true, ie. a selection must come from the list, it should be possible to also allow an empty selection if ng-required = false without triggering an error. Otherwise ng-required becomes ineffective.

Ie. I want the user to either:
- select something from the list
- leave the input empty, either by typing nothing or deleting what has been typed

If the prior behavior is needed, ie. the input has to be something in the list then ng-required should be set to true.