public Assignment targetAssignment(String memberId, String instanceId) {
        if (instanceId == null) {
            return targetAssignment(memberId);
        } else {
            String previousMemberId = staticMemberId(instanceId);
            if (previousMemberId != null) {
                return targetAssignment(previousMemberId);
            }
        }
        return Assignment.EMPTY;
    }
Returns the target assignment associated to the provided member id if
the instance id is null; otherwise returns the target assignment associated
to the instance id.

@param memberId      The member id.
@param instanceId    The instance id.

@return The Assignment or EMPTY if it does not exist.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/modern/consumer/ConsumerGroup.java