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

[Victor Chernyakin via cfe-commits]

localspook wrote:

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

Good point, added those. Apparently there was a bug where `std::string_view {NULL}` wouldn't be diagnosed without `-system-headers`, which I've fixed now.

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