generate nil safe library code

[iamneal]

protoc-gen-persist generates functions similar to this:

func AuthorToLibraryPersistType(req *Author) (*persist_lib.AuthorForLibrary, error) {
	params := &persist_lib.AuthorForLibrary{}
	params.Id = req.Id
	params.FirstName = req.FirstName
	params.LastName = req.LastName
	return params, nil
}

This is not safe if req is nil.