xmpp: set storage backend to "none"

"null" seems to have stopped working.

prosody/rootfs/defaults/conf.d/jitsi-meet.cfg.lua

@@ -22,7 +22,7 @@ VirtualHost "${XMPP_AUTH_DOMAIN}"
     authentication = "internal_plain"
 
 Component "${XMPP_MUC_DOMAIN}" "muc"
-    storage = "null"
+    storage = "none"
 
 Component "jitsi-videobridge.${XMPP_DOMAIN}"
     component_secret = "${JVB_COMPONENT_SECRET}"