feat(choices): add null option support

src/uiSelectChoicesDirective.js

@@ -26,6 +26,11 @@ uis.directive('uiSelectChoices',
 
         $select.disableChoiceExpression = attrs.uiDisableChoice;
         $select.onHighlightCallback = attrs.onHighlight;
+        // null-option support
+        if (angular.isDefined(attrs.nullOption)) {
+          $select.nullOption = scope.$eval(attrs.nullOption);
+          $select.looseNull = angular.isDefined(attrs.looseNull);
+        }
 
         if(groupByExp) {
           var groups = element.querySelectorAll('.ui-select-choices-group');

src/uiSelectDirective.js

@@ -121,6 +121,28 @@ uis.directive('uiSelect',
         return inputValue;
       });
 
+      //From view --> model (null-option support)
+      ngModel.$parsers.push(function (inputValue) {
+        // add a parser to convert the null option
+        // into an actual null value when it is selected
+        // (this should run after the main parser)
+        return angular.isDefined($select.nullOption) && inputValue === $select.nullOption ? null : inputValue;
+      });
+
+      //From model --> view (null-option support)
+      ngModel.$formatters.unshift(function (inputValue) {
+        // loose-null option (treat undefined the same as null)
+        var isNull = $select.looseNull ?
+                                /*jshint eqnull:true */
+                                function (value) { return value == null; } :
+                                function (value) { return value === null; };
+
+        // add a formatter to convert a null value
+        // into the null option when the model is set
+        // (this should run before the main formatter)
+        return angular.isDefined($select.nullOption) && isNull(inputValue) ? $select.nullOption : inputValue;
+      });
+
       //Set reference to ngModel from uiSelectCtrl
       $select.ngModel = ngModel;