Merge pull request #6965 from terry-xiaoyu/log_printable_mqtt_msg_id2

fix: emqx_message:format/1 failed on undefined message id

src/emqx_message.erl

@@ -342,9 +342,14 @@ format(#message{id = Id,
                 flags = Flags,
                 headers = Headers}) ->
     io_lib:format("Message(Id=~s, QoS=~w, Topic=~s, From=~p, Flags=~s, Headers=~s)",
-                  [emqx_guid:to_hexstr(Id), QoS, Topic, From, format(flags, Flags),
+                  [printable_msg_id(Id), QoS, Topic, From, format(flags, Flags),
                    format(headers, Headers)]).
 
+printable_msg_id(undefined) ->
+    <<>>;
+printable_msg_id(Id) ->
+    emqx_guid:to_hexstr(Id).
+
 format(flags, Flags) ->
     io_lib:format("~p", [[Flag || {Flag, true} <- maps:to_list(Flags)]]);
 format(headers, Headers) ->