fix(ngOptions): add ngEmptyValue attribute to ngOptions

allow an optional ng-empty-value attribute on select elements using ng-options. any option whose value is an empty string will be replaced with the given empty-value.

Reference: #13334

Author: Thesoro

src/ng/directive/ngOptions.js

@@ -489,6 +489,8 @@ var ngOptionsDirective = ['$compile', '$parse', function($compile, $parse) {
 
         selectCtrl.readValue = function readNgOptionsValue() {
 
+          if (selectElement.val() == '') return $parse(attr.ngEmptyValue)(scope);
+
           var selectedOption = options.selectValueMap[selectElement.val()];
 
           if (selectedOption && !selectedOption.disabled) {