fix(select): prevent unknown option being added to select when bound to null property

If a select directive was bound, using ng-model, to a property with a value of null this would
result in an unknown option being added to the select element with the value "? object:null ?".
This change prevents a null value from adding an unknown option meaning that the extra option is
not added as a child of the select element.

Since select (without ngOptions) can only have string value options then `null` was never a
viable option value, so this is not a breaking change.

Closes #11872
Closes #11875