ao_null: never fail at initialization

ao_null is used to stop autoprobing (if all AOs before fail to init).
After it come things like ao_pcm, which should never be automatically
selected.

Remove a certain theoretically possible failure case, and force "some"
fallback.

audio/out/ao_null.c

@@ -90,7 +90,7 @@ static int init(struct ao *ao)
     struct mp_chmap_sel sel = {0};
     mp_chmap_sel_add_any(&sel);
     if (!ao_chmap_sel_adjust(ao, &sel, &ao->channels))
-        return -1;
+        mp_chmap_from_channels(&ao->channels, 2);
 
     priv->latency = priv->latency_sec * ao->samplerate;