test_emptyness: discid_get_first_track_num: Assertion `disc->success' failed

Due to changes in libdiscid ([LIB-48](http://tickets.musicbrainz.org/browse/LIB-48)) the emptyness test fails:

```
test_emptyness (test_discid.TestClass) ... python: /home/jonnyjd/git/_musicbrainz/libdiscid/src/disc.c:236: discid_get_first_track_num: Assertion `disc->success' failed.
make: *** [check] Aborted (core dumped)
```

The test should be fixed in the 1.0 branch.