Crash when export with no arguments

The previous, verbose code actually crashed when for example someone forgets to put parenthesis:

```python
#pythran export test
def test():
    ...
```

In that case the `p` lex token passed to `p_error` is `None`, and so it crashes when trying to access `p.value`.

That crash was discovered by my own clumsiness yet again :o It's also a regression introduced by my previous commit on the matter.