pk=[] returns all articles

When using `list_articles()` one would expect the following code to run without any errors:

``` python
api = AmcatAPI(**auth)
articles = api.list_articles(project=pn, articleset=an, pk=[])
self.assertEqual(0, len(list(articles)))
```

However, leaving pk 'empty' results in filtering on no pk at all. This is unexpected behaviour.

@vanatteveldt How should we fix this, if at all?
