chore(gw-frame): export types for dialyzer check

apps/emqx_gateway/src/bhvrs/emqx_gateway_frame.erl

@@ -32,6 +32,12 @@
 
 -type serialize_options() :: map().
 
+-export_type([ parse_state/0
+             , parse_result/0
+             , serialize_options/0
+             , frame/0
+             ]).
+
 %% Callbacks
 
 %% @doc Initial the frame parser states

apps/emqx_gateway/src/coap/emqx_coap_channel.erl

@@ -235,7 +235,7 @@ handle_call({subscribe, Topic, SubOpts}, _From,
                          session = Session}) ->
     Token = maps:get(token,
                      maps:get(sub_props, SubOpts, #{}),
-                     undefined),
+                     <<>>),
     NSubOpts = maps:merge(
                  emqx_gateway_utils:default_subopts(),
                  SubOpts),