fix(ngSrcset): ignore undefined ng-srcset directive

Closes angular#14470

Author: rphv

Diff for: src/ng/compile.js

@@ -1447,7 +1447,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
             (nodeName === 'img' && key === 'src')) {
           // sanitize a[href] and img[src] values
           this[key] = value = $$sanitizeUri(value, key === 'src');
-        } else if (nodeName === 'img' && key === 'srcset') {
+        } else if (nodeName === 'img' && key === 'srcset' && isDefined(value)) {
           // sanitize img[srcset] values
           var result = "";
 

Diff for: test/ng/directive/ngSrcsetSpec.js

@@ -28,5 +28,12 @@ describe('ngSrcset', function() {
     $rootScope.$digest();
     expect(element.attr('srcset')).toBe('http://example.com/image1.png 1x,unsafe:javascript:doEvilStuff() 2x');
   }));
+
+  it('should not throw an error if undefined', inject(function($rootScope, $compile) {
+    $rootScope.imageUrl = {};
+    element = $compile('<img ng-srcset="{{undefined}}">')($rootScope);
+    $rootScope.$digest();
+    expect(element.attr('srcset')).toBe(undefined);
+  }));
 });