Do not construct std::string from nullptr
While I am trying to forbid such usages systematically in https://reviews.llvm.org/D79427 / P2166R0 to C++ standard, this PR fixes this (definitelly incorrect) usage in llvm.
This code is unreachable, so it could not cause any harm
Reviewed By: nikic, dblaikie
Differential Revision: https://reviews.llvm.org/D87697