internal/poll: don't skip empty writes on Windows

internal/poll: don't skip empty writes on Windows

Empty writes might be important for some protocols. Let Windows decide
what do with them rather than skipping them on our side. This is inline
with the behavior of other platforms.

While here, refactor the Read/Write/Pwrite methods to reduce one
indentation level and make the code easier to read.

Fixes #73084.

Change-Id: Ic5393358e237d53b8be6097cd7359ac0ff205309
Reviewed-on: https://go-review.googlesource.com/c/go/+/661435
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>