The Message.String method is intended for debugging,
so outputting <nil> for typed nil messages actually has use.
This matches the current behavior of Message.String
for messages generated by the v1 protoc-gen-go.
Change-Id: I6e31183961c83d7bce6338eb99aa7758cfda1ff4
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/199397
Reviewed-by: Damien Neil <dneil@google.com>