The v1 behavior of Clone returned nil as is.
Replicate this behavior in v2 for easier migration.
Change-Id: I79d93f15dd9913604027e31e5d9ff309e0c2da61
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/220437
Reviewed-by: Damien Neil <dneil@google.com>