Fix bug in episode toplist with empty episode actions
Fix bug in episode toplist with empty episode actions

When the episode actions table is empty, the episode
toplist failed, because the max() function did not
receive any parameters. This is now fixed by adding
a 0 value to the max parameter list.

gpodder/mygpo,gpodder/mygpo,gpodder/mygpo,gpodder/mygpo