Clone behavior on maps is _marginally_ different than vanilla Proto.Clone()

[gunnar-solo]

When playing around with protoc-gen-ext v0.0.16 and using option (extproto.clone_all) = true;, I've noticed an inconsitency from the generated object.Clone() methods vs. the previously-used proto.Clone(object) method. Specifically:

The nested field of UpstreamSpec.Selector changed how it is cloned in the default case. Where previously no provided value --> clone would equate to nil, now a clone generates an empty map: map[string]string{}.

While not a breaking (or important) change at the moment, I'm documenting this, as there may be (as of now) unforeseen consequences to changing default map values on clone.

Further (potentially better-written) context by @sam-heilbron: solo-io/gloo#5658 (comment)