[Fix] Fix various modules in case of empty message

src/plugins/lua/asn.lua

     type = 'prefilter,nostat',
     callback = asn_check,
     priority = 5,
+    flags = 'empty',
   })
   if options['symbol'] then
     rspamd_config:register_symbol({
       name = options['symbol'],
       parent = id,
-      type = 'virtual'
+      type = 'virtual',
+      flags = 'empty',
     })
   end
 else

src/plugins/lua/clickhouse.lua

         name = 'CLICKHOUSE_COLLECT',
         type = 'idempotent',
         callback = clickhouse_collect,
-        priority = 10
+        priority = 10,
+        flags = 'empty',
       })
       rspamd_config:register_finish_script(function(task)
         if nrows > 0 then

src/plugins/lua/elastic.lua

       name = 'ELASTIC_COLLECT',
       type = 'idempotent',
       callback = elastic_collect,
-      priority = 10
+      priority = 10,
+      flags = 'empty',
     })
 
     rspamd_config:add_on_load(function(cfg, ev_base,worker)

src/plugins/lua/force_actions.lua

                 type = 'normal',
                 name = name,
                 callback = cb,
+                flags = 'empty',
               })
               for _, a in ipairs(atoms) do
                 rspamd_config:register_dependency(id, a)
           end
           t.name = 'FORCE_ACTION_' .. name
           t.callback = cb
+          t.flags = 'empty'
           local id = rspamd_config:register_symbol(t)
           if t.type == 'normal' then
             for _, a in ipairs(atoms) do

src/plugins/lua/greylist.lua

       name = 'GREYLIST_SAVE',
       type = 'postfilter',
       callback = greylist_set,
-      priority = 6
+      priority = 6,
+      flags = 'empty',
     })
     rspamd_config:register_symbol({
       name = 'GREYLIST_CHECK',
       type = 'prefilter',
       callback = greylist_check,
-      priority = 6
+      priority = 6,
+      flags = 'empty',
     })
   end
 end

src/plugins/lua/history_redis.lua

       name = 'HISTORY_SAVE',
       type = 'idempotent',
       callback = history_save,
+      flags = 'empty',
       priority = 150
     })
     rspamd_plugins['history'] = {

src/plugins/lua/ip_score.lua

     type = 'postfilter,nostat',
     priority = 5,
     callback = ip_score_set,
+    flags = 'empty',
   })
   rspamd_config:register_symbol({
     name = options['symbol'],
     callback = ip_score_check,
     group = 'reputation',
-    score = 2.0
+    score = 2.0,
+    flags = 'empty',
   })
 else
   rspamd_lua_utils.disable_module(N, "redis")

src/plugins/lua/metadata_exporter.lua

     name = 'EXPORT_METADATA_' .. k,
     type = 'postfilter,idempotent',
     callback = gen_exporter(r),
-    priority = 10
+    priority = 10,
+    flags = 'empty',
   })
 end

src/plugins/lua/milter_headers.lua

   name = 'MILTER_HEADERS',
   type = 'postfilter,idempotent',
   callback = milter_headers,
-  priority = 10
+  priority = 10,
+  flags = 'empty',
 })

src/plugins/lua/mx_check.lua

     name = settings.symbol_bad_mx,
     type = 'normal',
     callback = mx_check,
+    flags = 'empty',
   })
   rspamd_config:register_symbol({
     name = settings.symbol_no_mx,

src/plugins/lua/settings.lua

       name = 'REDIS_SETTINGS' .. tostring(id),
       type = 'prefilter,nostat',
       callback = gen_redis_callback(h, id),
-      priority = 10
+      priority = 10,
+      flags = 'empty',
     })
   end, redis_key_handlers)
 end
   name = 'SETTINGS_CHECK',
   type = 'prefilter,nostat',
   callback = check_settings,
-  priority = 10
+  priority = 10,
+  flags = 'empty',
 })