fix(ngOptions): allow empty option selection with multiple attribute

Fixes angular#12511
Closes angular#12541

Author: arm1n

src/ng/directive/ngOptions.js

@@ -526,7 +526,7 @@ var ngOptionsDirective = ['$compile', '$parse', function($compile, $parse) {
 
           forEach(selectedValues, function(value) {
             var option = options.selectValueMap[value];
-            if (!option.disabled) selections.push(options.getViewValueFromOption(option));
+            if (option && !option.disabled) selections.push(options.getViewValueFromOption(option));
           });
 
           return selections;

test/ng/directive/ngOptionsSpec.js

@@ -2058,6 +2058,18 @@ describe('ngOptions', function() {
       // ensure the option has not changed following the digest
       expect(element[0].selectedIndex).toEqual(0);
     });
+
+
+    it('should be selectable if select is multiple', function() {
+      createMultiSelect(true);
+
+      // select the empty option
+      setSelectValue(element, 0);
+
+      // ensure selection and correct binding
+      expect(element[0].selectedIndex).toEqual(0);
+      expect(scope.selected).toEqual([]);
+    });
   });