Use null or undefined, not an empty string, as the "not selected" value.
That's what the docs say:
Optionally, a single hard-coded
element, with the value set to an empty string, can be nested into theelement. This element will then represent thenullor "not selected" option. See example below for demonstration.
It's not really clear that an empty string can't be used, but at least they mention null.