mapping of an empty array, error checking logic

I am new to react and a question on how to handle empty arrays. I have some data on a specific class. The class data can be seen below. I am trying to display the data in my web app but I have having trouble with the logic of mapping array when they are empty.

a simplified version of my code is shown below:

import React from 'react';
import { useState, useEffect } from "react";

const data = {
  ece1004: {
    number: "ECE 1004",
    name: "Introduction to ECE Concepts",
    des: "Introduction to topics that span the field of electrical and computer engineering (ECE). Content presented through the lens of application with accompanying hands-on exercises. Basics of circuits, op-amps, power supplies, computer logic, system decomposition, and coding. Modeling and application of engineering professionalism. Exploration of ECE in society.",
    pre: [["ENGE 1215", "ECE 1414"]],
    co: [],
    minGrade: "C",
    offering: ["Fall", "Spring", "Summer"],
  }
};

function ECE1004() { 
    const [data, setData]  = useState([]);

    const fetchClassObj = () => {
        setData(data);
    }
    useEffect(() => {
        fetchClassObj();
    },[]);

    return (
        <>   
            <div>
                {data.co ? 
                    data.co.map(coreq => {
                    return(      
                    <div className="classDes">
                        Coreqs: {coreq.join(', ')}
                    </div>
                    )
                }) : <h3>No data yet</h3> }
                
            </div>             
        </>
    )

}

export default ECE1004

I would like the class data to be displayed all in separate divs. For example, if I wrote the same code for the prereqs, it would display as “Prereqs: ECE 1215, ECE 1414” with a comma inbetween (hence the .join(', ') function).
However, since the co member of the data is empty the mapping function is not recognized. I was wondering what the logic is to check if the array is empty as well as use the .join(', ') function.
I have tried not mapping the data and using {data.co.length ? 'coreqs' + data.co.join(', ') : 'coreqs: None'} but then the .join(', ') is not recognized.
Any help would be appreciated, thank you.