bug(config): direct convert ret 0 to nil error

Author: kulti

spec/config_spec.lua

@@ -22,7 +22,7 @@ describe('config', function()
       remove = function() return 0 end,
       err_to_str = function(ret)
         if ret == 0 then
-          return ''
+          return 'everything is fine'
         elseif ret == 404 then
           return 'NotFound'
         elseif ret == 500 then

src/enapter/ucm/config.lua

@@ -4,6 +4,8 @@ local config = {}
 local function storage_err(v)
   if runtime_version == 'v3' then return v end
 
+  if v == 0 then return nil end
+
   local err = storage.err_to_str(v)
   if err == '' then err = nil end
   return err