fix html validator error

pages/forms/advanced.html

@@ -1473,7 +1473,7 @@
                 <div class="table table-striped files" id="previews">
                   <div id="template" class="row mt-2">
                     <div class="col-auto">
-                        <span class="preview"><img src="" alt="" data-dz-thumbnail /></span>
+                        <span class="preview"><img src="data:," alt="" data-dz-thumbnail /></span>
                     </div>
                     <div class="col d-flex align-items-center">
                         <p class="mb-0">