public boolean hasInitialMessage() {
        return !initialMembers.isEmpty();
    }
Returns true if the event stream has an initial-message, meaning
there are top-level members of the input or output of the operation
that are not the event stream member.

@return Returns true if there is an initial message.
smithy-model/src/main/java/software/amazon/smithy/model/knowledge/EventStreamInfo.java