`""` does not refer to a proper path, and `fs::PathStartsWith(path, "")` should just return `false`. This is also the case in libraries in other languages where I looked, seems to be common.
The new behavior:
* check early, if `prefix` is empty - return if path is empty or not,
* no special processing for when `path` is empty, the function meets characters in `prefix` and returns false anyway.
Use win32 APIs to write the temporary file before copying to the final
destination. Because we've observed the final file being empty, we
suspect that std::ostream::flush is not flushing.
Also add a test for it.
