Fallback-src doesn't set the fallback image if the src is empty

When the binding value for ng-src is null, src is not set and the fallback img is not set. This happens when loading the url from database and the url is not set for example.

``` js
$scope.someUrl = null;
...
<img ng-src='{{someUrl}}' fallback-src />
```

The fallback-src could check for the 'src' attribute and run the fallback handler if no src is set.
