Replace NULL and 0 with nullptr

https://google.github.io/styleguide/cppguide.html#0_and_nullptr/NULL says:
For pointers (address values), use nullptr, as this provides type-safety.

Additionally, NULL won't compile if we change |SomeClass*| to |CheckedPtr<SomeClass>|. See go/miracleptr for more details.

Bug: 1080832
Change-Id: I70f899fe45a31d55f98063585180686d2a256141
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2217946
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#773810}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 3253b67cc7b0dbc12e760a7d66584f8429023105
diff --git a/url_idna_icu.cc b/url_idna_icu.cc
index 0b0ffd7..8805678 100644
--- a/url_idna_icu.cc
+++ b/url_idna_icu.cc
@@ -52,7 +52,7 @@
                    << ". If you see this error message in a test environment "
                    << "your test environment likely lacks the required data "
                    << "tables for libicu. See https://crbug.com/778929.";
-      value = NULL;
+      value = nullptr;
     }
   }
 
@@ -84,7 +84,7 @@
   DCHECK(output->length() == 0);  // Output buffer is assumed empty.
 
   UIDNA* uidna = GetUIDNA();
-  DCHECK(uidna != NULL);
+  DCHECK(uidna != nullptr);
   while (true) {
     UErrorCode err = U_ZERO_ERROR;
     UIDNAInfo info = UIDNA_INFO_INITIALIZER;