Channel series card are broken

We miss a check for empty/null covers here https://dev.funkwhale.audio/funkwhale/funkwhale/-/blob/develop/front/src/components/audio/ChannelSerieCard.vue#L4-7

It makes the whole «series» block to crash on the channel detail page, like here: https://kirche.funkwhale-host.de/channels/sonntagsandacht/