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/wcstod.cpp b/libc/bionic/wcstod.cpp
index 41a94fb..75a59f5 100644
--- a/libc/bionic/wcstod.cpp
+++ b/libc/bionic/wcstod.cpp
@@ -62,7 +62,7 @@
   f._bf._base = f._p = reinterpret_cast<unsigned char*>(ascii_str);
   f._bf._size = f._r = max_len;
   f._read = [](void*, char*, int) { return 0; }; // aka `eofread`, aka "no more data".
-  f._lb._base = NULL;
+  f._lb._base = nullptr;
 
   // Ask `parsefloat` to look at the same data more carefully.