load all models if models arg is an empty string

My PR solves problem that I discovered when running the `sunspot:reindex` task with parameters. E.g.

```
rake sunspot:reindex[,,true]
```

This sets arguments:

```ruby
{:batch_size=>"", :models=>"", :silence=>"true"}
```

However, in such case, no reindexing can happen. `args[:models]` carry empty string that is evaluated as `true` on line 46. Because the string does contain any model names, no reindexing is then carried out.