Return number of bytes written in case of BlockingIOError.

In case of a partial write BufferedIOBase doesn't return the length of data written, but gives a BlockingIOError that contains the number of characters written.
See https://docs.python.org/3/library/io.html#io.BufferedIOBase.write

Also, RawIOBase returns None (and not 0) when no data can be written.

PiperOrigin-RevId: 268167212