fix(pdk.log): fix a bug that pdk.log.serialize() will throw an error

when json entity set via pdk.log.set_serialize_value contains cjson.null
as value.

FTI-6096

kong/pdk/log.lua

@@ -17,6 +17,7 @@ local inspect = require("inspect")
 local phase_checker = require("kong.pdk.private.phases")
 local constants = require("kong.constants")
 local clear_tab = require("table.clear")
+local cjson_null = require("cjson").null
 
 
 local request_id_get = require("kong.observability.tracing.request_id").get
@@ -640,7 +641,7 @@ do
 
   local function is_valid_value(v, visited)
     local t = type(v)
-    if v == nil or t == "number" or t == "string" or t == "boolean" then
+    if v == nil or v == cjson_null or t == "number" or t == "string" or t == "boolean" then
       return true
     end
 

spec/01-unit/10-log_serializer_spec.lua

@@ -231,6 +231,14 @@ describe("kong.log.serialize", function()
         assert.not_equal(tostring(ngx.ctx.service),
                          tostring(res.service))
       end)
+
+      it("does not fail when coming across 'cjson.null' in response body", function()
+        local cjson_null = require "cjson".null
+        kong.log.set_serialize_value("response.body", cjson_null)
+        local pok, value = pcall(kong.log.serialize, {})
+        assert.is_true(pok)
+        assert.is_true(type(value) == "table")
+      end)
     end)
   end)