fix(mqtt_bridge): ensure `server` key is a binary
Fixes https://emqx.atlassian.net/browse/EMQX-10461
So that it can be JSON encoded correctly.
```
2023-06-30T02:00:41.160110+00:00 [debug] msg: publish_to, mfa: emqx_trace:publish/1, line: 73, topic: b/$, payload: {"topic":"t/1","server":[49,48,46,52,50,46,51,46,49,56,48,58,49,56,56,51],"retain":false,"qos":1,"pub_props":{},"payload":"{\"msg\": \"hello\"}","message_received_at":1688090441159,"id":"0005FF4F2F181488103417000C2E0000","dup":false}, tag: PUBLISH
```
425eba8b13