fixup replay controller next side button

the replay controller `play next side` function was bugged because
it didn't check if the next side was empty, the way that the other
play controllers do. the fix is to make sure to repeat the process
while we are still on an empty side.

this fixes bug #21025
update changelogs
Merge branch '1.12' of https://github.com/wesnoth/wesnoth into 1.12