Handle empty og:image value

I found that some sites have empty og:image value, like below. It'd be nice to handle this edge case.

```html
<meta property="og:image" content="" name="">
```

But in `opengraph.rb` we're not checking empty string so `object.valid?`still returns true.

With this change `object[:images]` will remain blank in this case, and `object.valid?` will return false in strict mode.