perf: keep defaults of zone.mqtt

apps/emqx/src/emqx_zone_schema.erl

@@ -61,6 +61,9 @@ hidden() ->
         "flapping_detect"
     ].
 
+%% optimized for mqtt field
+fields("mqtt") ->
+    emqx_schema:fields("mqtt");
 %% zone schemas are clones from the same name from root level
 %% only not allowed to have default values.
 fields(Name) ->