[ADT] Use std::nullopt instead of llvm::None in OptionalTest.cpp

This patch updates the test for the streaming operator for
llvm::Optional to use std::nullopt instead of llvm::None.

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

Differential Revision: https://reviews.llvm.org/D139513