fwrite() is valid to call with a 0 count, and will simply return 0.
See:
https://en.cppreference.com/w/cpp/io/c/fwritehttp://www.cplusplus.com/reference/cstdio/fwrite/
There are code paths where StreamWriter will call Tell(), which calls
Flush(), which calls Write(buffer.data(), 1, buffer.size()). This can
happen when nothing has yet been written to the buffer, so size is 0.