[clang-tidy] Use nullptr keyword

Recommended by clang-tidy's modernize-use-nullptr.

Bug: v8:10488
Change-Id: I7cf26f7bda69b9b1a6bdabe274399b494f6d49ed
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2211155Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67979}

src/objects/intl-objects.h

@@ -287,7 +287,7 @@ class Intl {
 
       std::vector<std::string> all_locales;
       const char* loc;
-      while ((loc = uenum_next(uenum, NULL, &status)) != nullptr) {
+      while ((loc = uenum_next(uenum, nullptr, &status)) != nullptr) {
         DCHECK(U_SUCCESS(status));
         std::string locstr(loc);
         std::replace(locstr.begin(), locstr.end(), '_', '-');