[llvm] Fix llvm::StripTemplateParameters to not return an empty name. (PR #157553)

[Michael Buch via llvm-commits]

Michael137 wrote:

Instead of adding this special-case, could we just check if the stripped name is empty, and if so return `std::nullopt`? That we guarantee that this function returns `std::nullopt` or a non-empty string.

https://github.com/llvm/llvm-project/pull/157553