Replace None with std::nullopt in comments (NFC)

This is part of an effort to migrate from llvm::Optional to
std::optional:

https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716

GitOrigin-RevId: 81e149aab9b3676b16728e883e2f21ee938cff93
1 file changed