You can first check if get_gallery_image_ids returns an array. If it exists, check if key 0 (the first element) exists. If so, then you're free to use it however you want.

...

// Get all IDs
$idList = wc_get_product()->get_gallery_image_ids(); 

// Check if the IDs are an array and key 0 (first element) exists
if (is_array($idList) && array_key_exists(0, $idList)) {
    // Get the first element
    $image_id = $idList[0];

    echo wp_get_attachment_image($image_id, 'woocommerce_thumbnail' ) ; 
} else { 
    echo wp_get_attachment_image(wc_get_product()->get_image_id(), 'woocommerce_thumbnail' ) ; 
}

edit,

You should edit your mem_add_on_hover_shop_loop_image function with this code. The final code should look like this,

add_action('woocommerce_before_shop_loop_item_title', 'mem_add_on_hover_shop_loop_image');
function mem_add_on_hover_shop_loop_image()
{
    // Get all IDs
    $idList = wc_get_product()->get_gallery_image_ids();

    // Check if the IDs are an array and key 0 (first element) exists
    if (is_array($idList) && array_key_exists(0, $idList)) {
        // Get the first element
        $image_id = $idList[0];

        echo wp_get_attachment_image($image_id, 'woocommerce_thumbnail');
    } else {
        echo wp_get_attachment_image(wc_get_product()->get_image_id(), 'woocommerce_thumbnail');
    }
}