Fix null pointer exception if song has neither artist nor album information.

Fix null pointer exception if song has neither artist nor album information.

For songs that we couldn't determine any information on artist and album for we used
to crash with a null pointer exception, since in this particular case (audio file
attachment received via email and opened using music app, not even song title
information was available we would have ended up showing a popup
"Search for null/'' using ..." which I consider unhelpful and futile enough
to just disable this case altogether.

(bug 1785479)