bufio: don't do empty Write at start of WriteTo

bufio: don't do empty Write at start of WriteTo

The empty Write will cause the wrong thing to happen when using
io.Copy to copy to a package-based stream.

Fixes #71424

Change-Id: I046a27539447182692ac76a8bdd422327345dd8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/644535
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>