fix(ngHref): allow setting href to empty string

Fixed issue 2755:
https://github.com/angular/angular.js/issues/2755

ngHref should now allow empty strings, and ngSrc will not accept empty strings as intended: https://github.com/angular/angular.js/commit/b6e4a71166c7f00f4140fd7ea8f0cd81b4487a3f