file_util,GetTempFileName: fix return value

`nullptr` -> `std::string()`; constructing a string from a `nullptr` is
undefined behavior:

https://clang.llvm.org/extra/clang-tidy/checks/bugprone/string-constructor.html

Change-Id: I5d57084decc92f0e8c55c4e2c9553e2ac83d5575