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/fortify_filecheck_diagnostics_test.cpp b/tests/fortify_filecheck_diagnostics_test.cpp
index 375a156..5629408 100644
--- a/tests/fortify_filecheck_diagnostics_test.cpp
+++ b/tests/fortify_filecheck_diagnostics_test.cpp
@@ -211,7 +211,7 @@
   // NOLINTNEXTLINE(whitespace/line_length)
   // GCC: error: call to '__recvfrom_error' declared with attribute error: 'recvfrom' called with size bigger than buffer
   // CLANG: error: 'recvfrom' called with size bigger than buffer
-  recvfrom(0, buf, 6, 0, reinterpret_cast<sockaddr*>(&addr), NULL);
+  recvfrom(0, buf, 6, 0, reinterpret_cast<sockaddr*>(&addr), nullptr);
 }
 
 void test_recv() {
@@ -398,9 +398,9 @@
   realpath(".", buf);
 
   // This is fine.
-  realpath(".", NULL);
+  realpath(".", nullptr);
 
   char bigbuf[PATH_MAX];
   // CLANG: error: 'realpath': NULL path is never correct; flipped arguments?
-  realpath(NULL, bigbuf);
+  realpath(nullptr, bigbuf);
 }