fix(mqtt-bridge): transmit raw msg payload with empty template

apps/emqx_connector/src/mqtt/emqx_connector_mqtt_msg.erl

@@ -71,14 +71,13 @@ to_remote_msg(#message{flags = Flags0} = Msg, Vars) ->
 to_remote_msg(MapMsg, #{
     remote := #{
         topic := TopicToken,
-        payload := PayloadToken,
         qos := QoSToken,
         retain := RetainToken
-    },
+    } = Remote,
     mountpoint := Mountpoint
 }) when is_map(MapMsg) ->
     Topic = replace_vars_in_str(TopicToken, MapMsg),
-    Payload = process_payload(PayloadToken, MapMsg),
+    Payload = process_payload(Remote, MapMsg),
     QoS = replace_simple_var(QoSToken, MapMsg),
     Retain = replace_simple_var(RetainToken, MapMsg),
     PubProps = maps:get(pub_props, MapMsg, #{}),
@@ -100,16 +99,15 @@ to_broker_msg(
     #{
         local := #{
             topic := TopicToken,
-            payload := PayloadToken,
             qos := QoSToken,
             retain := RetainToken
-        },
+        } = Local,
         mountpoint := Mountpoint
     },
     Props
 ) ->
     Topic = replace_vars_in_str(TopicToken, MapMsg),
-    Payload = process_payload(PayloadToken, MapMsg),
+    Payload = process_payload(Local, MapMsg),
     QoS = replace_simple_var(QoSToken, MapMsg),
     Retain = replace_simple_var(RetainToken, MapMsg),
     PubProps = maps:get(pub_props, MapMsg, #{}),
@@ -121,9 +119,12 @@ to_broker_msg(
         )
     ).
 
-process_payload([], Msg) ->
+process_payload(From, MapMsg) ->
+    do_process_payload(maps:get(payload, From, undefined), MapMsg).
+
+do_process_payload(undefined, Msg) ->
     emqx_json:encode(Msg);
-process_payload(Tks, Msg) ->
+do_process_payload(Tks, Msg) ->
     replace_vars_in_str(Tks, Msg).
 
 %% Replace a string contains vars to another string in which the placeholders are replace by the

changes/v5.0.14/fix-9672.en.md

@@ -0,0 +1 @@
+Fix the problem that the bridge is not available when the Payload template is empty in the MQTT bridge.

changes/v5.0.14/fix-9672.zh.md

@@ -0,0 +1 @@
+修复 MQTT 桥接中 Payload 模板为空时桥接不可用的问题。