clang-tidy: Replace NULL with nullptr

Replaced all instances of NULL with nullptr to improve type safety
and clarity, as nullptr is the modern C++ standard for null pointers.

Tested: Build verified

Change-Id: I05ae9b6de8e2e857e1951e5d250e2fa4afc0c634
Signed-off-by: Jayanth Othayoth <ojayanth@gmail.com>
diff --git a/nvme_manager.cpp b/nvme_manager.cpp
index 7270e9f..7a64a25 100644
--- a/nvme_manager.cpp
+++ b/nvme_manager.cpp
@@ -102,7 +102,7 @@
                                      NVME_STATUS_IFACE, "SmartWarnings",
                                      nvmeData.smartWarnings);
         auto smartWarning = (!nvmeData.smartWarnings.empty())
-                                ? std::stoi(nvmeData.smartWarnings, 0, 16)
+                                ? std::stoi(nvmeData.smartWarnings, nullptr, 16)
                                 : NOWARNING;
 
         util::SDBusPlus::setProperty(bus, INVENTORY_BUSNAME, inventoryPath,