--stop-playback-on-init-failure=no is documented to continue playback in
audio-only mode if video init fails. However, this does not function
properly after a0804329927904fdeb70d9712ff23baaab161bb4: because video
is initialized first, if video init fails, ao_chain is still null.
As a result, !(mpctx->vo_chain || mpctx->ao_chain) is true, a playback
error is raised, so the playback ends and audio does not play.
Fix this by reverting that change, so it checks the tracks instead.
Fixes: a080432992