[cleanup] Replace uses of NULL with nullptr

Generated using clang-tidy,

tools/clang/scripts/build_clang_tools_extra.py \
  --fetch out/Default clang-tidy clang-apply-replacements
ninja -C out/Default
gn gen out/Default --export-compile-commands
cd out/Default
tools/clang/third_party/llvm/clang-tools-extra/clang-tidy/tool/run-clang-tidy.py -p . \
  -clang-tidy-binary out/Default/tools/clang/third_party/llvm/build/bin/clang-tidy \
  -clang-apply-replacements-binary \
      out/Default/tools/clang/third_party/llvm/build/bin/clang-apply-replacements \
  -checks='-*,modernize-use-nullptr' \
  -fix

This CL was uploaded by git cl split.

Bug: webrtc:42232237
Change-Id: Iba6258b9c83bf8623fc54f483ab72824ead462ef
No-Iwyu: LSC
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/391642
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Per Åhgren <peah@webrtc.org>
Auto-Submit: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#44692}