String optional(@Nonnull String str) {
        return str.isEmpty() ? null : str;
    }
Convert a non-null String instance to a possibly null String instance based on ProtoBuf's
rules for optional arguments.

<p>This helper converts an String instance from a ProtoBuf object into a possibly null
String. In ProtoBuf objects, String fields are not nullable, but an empty String field is
considered to be "missing".

<p>The internal Conductor APIs expect missing arguments to be passed as null values, so this
helper performs such conversion.

@param str a string from a ProtoBuf object
@return the original string, or null
grpc-server/src/main/java/com/netflix/conductor/grpc/server/service/GRPCHelper.java