picard crashes when matching a cluster with a release with no tracks

When trying to match a cluster with the release at

https://musicbrainz.org/release/c8b5f0e6-96d9-4160-a1d9-ddcf005d4482

picard raised an exception (and crashed) because 'track-count' and 'media' were not defined in the release object. 

The exception was raised at lines:

https://github.com/metabrainz/picard/pull/915/commits/717be2627cd70997f754477325a2f949546e8565#diff-d8e66386616f4db41b290d95ffc71ef2L156

and also, when that one was fixed, at:

https://github.com/metabrainz/picard/pull/915/commits/717be2627cd70997f754477325a2f949546e8565#diff-d8e66386616f4db41b290d95ffc71ef2L178