[Minor] Fix debugm logic

lualib/lua_util.lua

@@ -750,7 +750,7 @@ exports.debugm = function(mod, obj_or_fmt, fmt_or_something, ...)
   local logger = require "rspamd_logger"
   if unconditional_debug or debug_modules[mod] then
     if type(obj_or_fmt) == 'string' then
-      logger.logx(log_level, mod, 2, obj_or_fmt, fmt_or_something, ...)
+      logger.logx(log_level, mod, '', 2, obj_or_fmt, fmt_or_something, ...)
     else
       logger.logx(log_level, mod, obj_or_fmt, 2, fmt_or_something, ...)
     end