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

[divercraig]

…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

Closes #11872

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