chore(exhook): fix diaylzer warnings

apps/emqx_exhook/src/emqx_exhook_handler.erl

@@ -269,8 +269,6 @@ message(#message{id = Id, qos = Qos, from = From, topic = Topic,
       headers => headers(Headers)
      }.
 
-headers(undefined) ->
-    #{};
 headers(Headers) ->
     Ls = [username, protocol, peerhost, allow_publish],
     maps:fold(