net/http: allow Handlers to test Hijacked conn without spamming error log

net/http: allow Handlers to test Hijacked conn without spamming error log

Make a zero-byte write to a hijacked connection not log anything, so handlers
can test whether a connection is hacked by doing a Write(nil).

Fixes #16456

Change-Id: Id56caf822c8592067bd8422672f0c1aec89e866c
Reviewed-on: https://go-review.googlesource.com/30812
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>