database/sql: optimized []byte copy + []byte(nil) -> *interface fix

Make the copy directly in the convert switch instead of an extra loop.
Also stops converting nil-[]byte to zero-[]byte when assigning to *interface

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7962044

Committer: Brad Fitzpatrick <bradfitz@golang.org>