correctly transform not applicable values for edition

Hi!

```
>>> from cpe.cpe import CPE
>>> CPE('wfn:[part="a", vendor="TauPan", product="cpelib", version="1\\.0\\.5", update=NA, edition=NA]').as_uri_2_3()
```

Throws an exception AttributeError: 'CPEComponentNotApplicable' object has no attribute 'as_uri_2_3', which my patch fixes.