When an invalid model value (eg empty string) was preset ngModel on
select[multiple] would throw an error, which is inconsistent with how it
works on other user input elements. Setting the model value to null or
undefined would also have no effect on what was already selected in the
UI. Fix this by clearing selected options when model set to null,
undefined or a type other than Array.
Closes#11926
7b0a86718c