Is that the only UL tag in the document?

If yes, then you can use the JS Function getElementsByTagName()

Check the code below, if it meets your requirement.

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "">
<html xmlns="">
  <title> new document </title>
  <script type="text/javascript">
		function  checkFeedCount(){
			var x1=document.getElementsByTagName('ul');
			var x2=document.getElementsByTagName('li');
			if (x2.length < 1){
				x1[0].innerHTML="<li>There are no announcements at this time.  Check back later!</li>";
 <body onLoad="checkFeedCount()">
  <div class="rss-box"><ul class="rss-items"></ul></div>