Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: I5b4123bc6709641315120a191e36cc57541349b2
diff --git a/tests/libs/dlopen_testlib_ifunc_variable_impl.cpp b/tests/libs/dlopen_testlib_ifunc_variable_impl.cpp
index a550fef..4b13eba 100644
--- a/tests/libs/dlopen_testlib_ifunc_variable_impl.cpp
+++ b/tests/libs/dlopen_testlib_ifunc_variable_impl.cpp
@@ -49,5 +49,5 @@
 
 extern "C" void* foo_ifunc() {
    char* choice = getenv("IFUNC_CHOICE");
-   return choice == NULL ? &v1 : &v2;
+   return choice == nullptr ? &v1 : &v2;
 }