audio: add option for falling back to ao_null

The manpage entry explains this.

(Maybe this option could be always enabled and removed. I don't quite
remember what valid use-cases there are for just disabling audio
entirely, other than that this is also needed for audio decoder init
failure.)

audio/out/ao.c

@@ -240,6 +240,7 @@ static void split_ao_device(void *tmp, char *opt, char **out_ao, char **out_dev)
 }
 
 struct ao *ao_init_best(struct mpv_global *global,
+                        bool ao_null_fallback,
                         struct input_ctx *input_ctx,
                         struct encode_lavc_context *encode_lavc_ctx,
                         int samplerate, int format, struct mp_chmap channels)
@@ -282,6 +283,11 @@ struct ao *ao_init_best(struct mpv_global *global,
         }
     }
 
+    if (ao_null_fallback) {
+        MP_TARRAY_APPEND(tmp, ao_list, ao_num,
+            (struct m_obj_settings){.name = "null"});
+    }
+
     for (int n = 0; n < ao_num; n++) {
         struct m_obj_settings *entry = &ao_list[n];
         bool probing = n + 1 != ao_num;