fix subscriptions

src/handlers/handler_utils.erl

@@ -319,8 +319,13 @@ subscription_to_json(Subscription) ->
 %%%===================================================================
 
 %% @private
--spec datetime_to_iso8601(calendar:datetime() | undefined) -> binary() | undefined.
+-spec datetime_to_iso8601(calendar:datetime() | binary() | undefined) -> binary() | undefined.
 datetime_to_iso8601(undefined) -> undefined;
+datetime_to_iso8601(Bin) when is_binary(Bin) ->
+  case byte_size(Bin) > 0 of
+    true -> Bin;
+    false -> undefined
+  end;
 datetime_to_iso8601({{Year, Month, Day}, {Hour, Minute, Second}}) ->
   iolist_to_binary(
     io_lib:format("~4..0B-~2..0B-~2..0BT~2..0B:~2..0B:~2..0BZ",