searchqueryset.all() return empty list when no input.

I want to get all results when there is no input.
So I add below code to my search form.It returns a empty list.

```
    def no_query_found(self):
        return self.searchqueryset.all()
```

It returns all when I exclude a special char.

```
    def no_query_found(self):
        return self.searchqueryset.exclude(content='foo')
```

env: win 7,  django 1.4 , haystack 2.1.0, whoosh 2.5.7
