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/system_properties/system_properties.cpp b/libc/system_properties/system_properties.cpp
index d5c3647..d7c441b 100644
--- a/libc/system_properties/system_properties.cpp
+++ b/libc/system_properties/system_properties.cpp
@@ -214,7 +214,7 @@
 int SystemProperties::Get(const char* name, char* value) {
   const prop_info* pi = Find(name);
 
-  if (pi != 0) {
+  if (pi != nullptr) {
     return Read(pi, nullptr, value);
   } else {
     value[0] = 0;