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/atof.cpp b/libc/bionic/atof.cpp
index e77ccdd..c17deab 100644
--- a/libc/bionic/atof.cpp
+++ b/libc/bionic/atof.cpp
@@ -31,5 +31,5 @@
 double atof(const char* s) {
   // Despite the 'f' in the name, this returns a double and is
   // specified to be equivalent to strtod.
-  return strtod(s, NULL);
+  return strtod(s, nullptr);
 }