fix(typeahead): don't throw ReferenceError when viewValue is undefined

When the typeahead's model value isn't initialized somewhere, the $isVisible 
method would throw because ngModel.$viewValue is undefined. This change will 
consider the value to be below the min length if the view value is undefined.

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Feel free to re-organize the test or adjust style or whatever. As far as I can
tell, typeahead is the only module affected by this, since there's no way to
set the minLength option with the bs-select directive. The datepicker might
also be affected, though.