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.cpp b/tests/libs/dlopen_testlib_ifunc.cpp
index f8acba7..f2e0025 100644
--- a/tests/libs/dlopen_testlib_ifunc.cpp
+++ b/tests/libs/dlopen_testlib_ifunc.cpp
@@ -61,7 +61,7 @@
 
 extern "C" fn_ptr foo_ifunc() {
    char* choice = getenv("IFUNC_CHOICE");
-   return choice == NULL ? f1 : f2;
+   return choice == nullptr ? f1 : f2;
 }
 
 extern "C" const char* foo_library() {