fix: work-around missing entries in playlist objects

Tank only returns an entries property for playlist objects if the
entries list would have at least one item. This is a bug [1].

closes #119

[1] tank#46

src/components/shows/PlaylistSelector.vue

@@ -264,7 +264,7 @@ export default {
       const totalDuration = this.playlistDuration(playlist.item)
       let delta = 0
 
-      const unknowns = playlist.item.entries.filter((entry) => !entry.duration)
+      const unknowns = (playlist.item.entries ?? []).filter((entry) => !entry.duration)
       if (unknowns.length === 1) {
         delta = this.timeslotDurationInNs - totalDuration
       }