oaiharvester: refextract task fix

* Fixes the retrieval of the fulltext.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
I see. I propose then to keep the code as is and add this to line 322:

```python
if pdf:
     obj.extra_data["_result"]["pdf"] = pdf
```

It is true that this is kind of duplicated across all postprocess task that require a PDF to be present. Can you check these functions that they are not doing the same mistake? (See for example `author_list` etc.)