Modernize codebase by replacing NULL with nullptr

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

Test: m
Bug: 68236239
Change-Id: I5b4123bc6709641315120a191e36cc57541349b2
diff --git a/libc/bionic/sysinfo.cpp b/libc/bionic/sysinfo.cpp
index 947de95..1d1070e 100644
--- a/libc/bionic/sysinfo.cpp
+++ b/libc/bionic/sysinfo.cpp
@@ -53,7 +53,7 @@
 
   int result = 0;
   dirent* entry;
-  while ((entry = reader.ReadEntry()) != NULL) {
+  while ((entry = reader.ReadEntry()) != nullptr) {
     if (entry->d_type == DT_DIR && __matches_cpuN(entry->d_name)) {
       ++result;
     }