clang-tidy: Enable modernize-use-nullptr check

This check converts the usage of null pointer constants
(e.g. NULL, 0) to use the new C++11 and C23 nullptr keyword.

Signed-off-by: George Liu <liuxiwei@ieisystem.com>
Change-Id: I10dccfd0513385c583ca07d13ed71edb04b58853
diff --git a/manager/lamptest/lamptest.hpp b/manager/lamptest/lamptest.hpp
index c7804c8..5e312e2 100644
--- a/manager/lamptest/lamptest.hpp
+++ b/manager/lamptest/lamptest.hpp
@@ -39,7 +39,7 @@
      */
     LampTest(const sdeventplus::Event& event, Manager& manager) :
         timer(event, [this](auto&) { timeOutHandler(); }), manager(manager),
-        groupObj(NULL)
+        groupObj(nullptr)
     {
         // Get the force update and/or skipped physical LEDs names from the
         // lamp-test-led-overrides.json file during lamp