[clang-tools-extra] [clang-tidy] support expect no diagnosis test (PR #91293)

[Congcong Cai via cfe-commits]

HerrCai0907 wrote:

> And why is this not a valid test? No `CHECK-*` comments in the reference file, so no violations from `clang-tidy` are expected?

Because the FileCheck don't support empty input.

https://github.com/llvm/llvm-project/pull/91293