When a map or slice is used as a return type create an empty value
rather than a nil value.

Fixes #19588

Change-Id: I577fd74956172329745d614ac37d4db8f737efb8
Reviewed-on: https://go-review.googlesource.com/38474
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>