Send a list containing "unkown" to summary.html if values are unkown

summary.html expects artist, title, release and track number to be
lists. If any of those tags is not a list, it will still access the
first element for printing, which, in the case of unkown values, was
just "[".

An example track is http://acousticbrainz.org/fe65b13e-6edc-40b8-a022-7a64136c9458