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: Iade4e4f1f25092f880fe06d61703db505b6e03d1
Signed-off-by: Jayanth Othayoth <ojayanth@gmail.com>
diff --git a/presence/gpio_presence.hpp b/presence/gpio_presence.hpp
index 3125ba2..52a1653 100644
--- a/presence/gpio_presence.hpp
+++ b/presence/gpio_presence.hpp
@@ -81,7 +81,7 @@
             // DRIVER_BIND_DELAY_MS environment variable is set.
             // Update the bind delay (in milliseconds) to the value from the
             // environment.
-            delay = std::strtoull(envDelay, NULL, 10);
+            delay = std::strtoull(envDelay, nullptr, 10);
         }
         determinePresence();
     }