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/jemalloc_wrapper.cpp b/libc/bionic/jemalloc_wrapper.cpp
index 19081a4..812884c 100644
--- a/libc/bionic/jemalloc_wrapper.cpp
+++ b/libc/bionic/jemalloc_wrapper.cpp
@@ -25,7 +25,7 @@
   size_t pagesize = getpagesize();
   size_t size = __BIONIC_ALIGN(bytes, pagesize);
   if (size < bytes) {
-    return NULL;
+    return nullptr;
   }
   return je_memalign(pagesize, size);
 }