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

[Victor Chernyakin via cfe-commits]

localspook wrote:

For reference, we only have 3 checks (excluding this one) using the Transformer API:
* abseil-string-find-str-contains
* abseil-cleanup-ctad
* llvm-use-new-mlir-op-builder

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