[clang-tidy] Use nullptr keyword

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

Bug: v8:10488
Change-Id: I03a6af87f281c52edd01f3ad91f9ec6e28d398d0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2233985Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68302}

src/base/platform/platform-posix.cc

@@ -531,7 +531,7 @@ OS::MemoryMappedFile* OS::MemoryMappedFile::open(const char* name,
 OS::MemoryMappedFile* OS::MemoryMappedFile::create(const char* name,
                                                    size_t size, void* initial) {
   if (FILE* file = fopen(name, "w+")) {
-    if (size == 0) return new PosixMemoryMappedFile(file, 0, 0);
+    if (size == 0) return new PosixMemoryMappedFile(file, nullptr, 0);
     size_t result = fwrite(initial, 1, size, file);
     if (result == size && !ferror(file)) {
       void* memory = mmap(OS::GetRandomMmapAddr(), result,

src/snapshot/snapshot-utils.cc

@@ -17,7 +17,7 @@ uint32_t Checksum(Vector<const byte> payload) {
   MSAN_MEMORY_IS_INITIALIZED(payload.begin(), payload.length());
 #endif  // MEMORY_SANITIZER
   // Priming the adler32 call so it can see what CPU features are available.
-  adler32(0, NULL, 0);
+  adler32(0, nullptr, 0);
   return static_cast<uint32_t>(adler32(0, payload.begin(), payload.length()));
 }