[clang-tools-extra] [clang-tidy] Speed up/rewrite `bugprone-stringview-nullptr` (PR #192889)

[Daniil Dudkin via cfe-commits]

https://github.com/unterumarmung commented:

> The check now uses the same mechanism as -Wnonnull, which is more powerful (before, it couldn't even flag std::string_view {0}, and the docs recommended running modernize-use-nullptr beforehand).

I think, some test cases for `0`, `NULL` should be added




https://github.com/llvm/llvm-project/pull/192889