You can check to see if the first childNode of the .ca parent element has content, and then target .aa from there if it does not:

//If there is no content within the first div of the class="ca" div...
//choose between textContent and innerHTML below 
//based on whether it is going to have any text or some HTML within (with possibly no text)
if (!document.querySelector('.ca').childNodes[0].innerHTML) {
    //hide the class="aa" div
    document.querySelector('.aa').style.display = 'none';
}

You can use children(":first") to get first child of div as

$(document).ready(function(){
if($('.ca').children(":first").text() == ""){
  $('.aa').hide();
}
});

$(document).ready(function(){
if($('.ca').children(":first").text() == ""){
  $('.aa').hide();
}
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="aa">
<div class="ca">
   <div></div>
   <div class="cb">A</div>
   <div class="cc">B</div>
</div>
<div>