one more placeholder case for blank src attribute

In some templates, such as ERB for Rails, it will automatically generate a `<img>` tag likes:
``` html
<img class="lazy" data-original="something.jpg" height="180" src="" width="180">
```
so the plugin will not work because the `if` in https://github.com/tuupola/jquery_lazyload/blob/master/jquery.lazyload.js#L92
``` js
if ($self.attr("src") === undefined || $self.attr("src") === false)  {
```
will always be false, and the placeholder will not be shown.

I append a new case which considers a blank `src` attribute like `''`.