Handle broken artist entries

e.g. as in http://www.bbc.co.uk/programmes/b04sn1n6/segments.inc
(Specifically the "Hot Music" entry.)

There is an artist entry, but no text inside it which then breaks due
to artists = artist_names[0].
Merge pull request #8 from StevenMaude/handle-broken-artist-entries

Handle broken artist entries