fix(Selectize): hide input box when selected data is 0

Previous check was only for a falsy value rather than null or undefined
causing input box not to be hidden even though an item had been selected.
(As that item's value evaluated to fals-y) 

Update check to ensure falsy-values actually register as being selected.

Closes angular-ui#1304

src/selectize/select.tpl.html

@@ -8,7 +8,7 @@
            ng-click="$select.toggle($event)"
            placeholder="{{$select.placeholder}}"
            ng-model="$select.search"
-           ng-hide="!$select.searchEnabled || ($select.selected && !$select.open)"
+           ng-hide="!$select.searchEnabled || (!$select.isEmpty() && !$select.open)"
            ng-disabled="$select.disabled"
            aria-label="{{ $select.baseTitle }}">
   </div>