[libFuzzer] Make -stop_file work with empty files (#220641) Fixes: https://github.com/llvm/llvm-project/issues/72334. The -stop_file flag is documented as "Stop fuzzing ASAP if this file exists", but the fuzzing loop checked it by reading the file and testing that the result is non-empty. Instead, check for the file's existence with IsFile. NOKEYCHECK=True GitOrigin-RevId: 6ed299c32b9d23be93d076d476c4d9a661ed6006