Fix lua delgd test: use sentinel default instead of nil comparison

cjson decodes JSON null as cjson.null, not Lua nil, so == nil always fails.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/validation/items/lua_func/lua_func.lua

@@ -45,7 +45,7 @@ function module.test_delgd()
     tm.setgd("_lua_delgd_test", 42)
     assert(tm.gd("_lua_delgd_test") == 42)
     tm.delgd("_lua_delgd_test")
-    assert(tm.gd("_lua_delgd_test") == nil)
+    assert(tm.gd("_lua_delgd_test", "__deleted__") == "__deleted__")
     return 0
 end