This CL expands upon a change made in (http://golang.org/cl/21811)
to ensure that a nil RawMessage gets serialized as "null" instead of
being a nil slice.

The added check only triggers when the RawMessage is nil. We do not
handle the case when the RawMessage is non-nil, but empty.

Fixes #17704
Updates #14493

Change-Id: I0fbebcdd81f7466c5b78c94953afc897f162ceb4
Reviewed-on: https://go-review.googlesource.com/32472Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>