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

Author: petebacondarwin

src/ng/directive/select.js

@@ -316,7 +316,7 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
             selectElement.val(viewValue);
             if (viewValue === '') emptyOption.prop('selected', true); // to make IE9 happy
           } else {
-            if (isUndefined(viewValue) && emptyOption) {
+            if (viewValue == null && emptyOption) {
               selectElement.val('');
             } else {
               selectCtrl.renderUnknownOption(viewValue);

test/ng/directive/selectSpec.js

@@ -362,7 +362,7 @@ describe('select', function() {
         scope.$apply(function() {
           scope.robot = null;
         });
-        expect(element).toEqualSelect(['? object:null ?'], '', 'c3p0', 'r2d2');
+        expect(element).toEqualSelect([''], 'c3p0', 'r2d2');
 
         scope.$apply(function() {
           scope.robot = 'r2d2';