refactor: NULL => nullptr

[scivision]

this was done with IDE find and replace NULL => nullptr
To me, there were too many lints that had accumulated to use clang-tidy directly.

if this makes a PR conflict, it may only be on a couple lines, and they could likewise do find/replace.

[digit-google]

This is nice! I would suggest two small things though:

1. Provide clear instructions on how to reproduce this locally, so that developers can easily update their own patches on top of this change once it is submitted.

2. Apply git clang-format just after the clang-tidy step, and add that in the commit message as well.

For example, on my Debian/testing machine, I could do the following:

clang-tidy-14 --checks=-*,modernize-use-nullptr --fix-errors src/*.cc src/*.h -- std=c++11
git clang-format

Using "-*" at the start of --checks is necessary to disable all the checks in the .clang-tidy file, since there are tons of unrelated errors trigerred by the source code if I do not use this.

[jhasse]

The change to .clang-tidy seems to be gone?

Also can you edit lexer.cc and disk_interface.cc without the whitespace changes?