correct default value for compounds list text search

Former-commit-id: 0467d19aebd7f9f35ffb79d53fc23164737759bb

ippisite/ippidb/templates/compound_list.html

@@ -30,7 +30,7 @@
             <span class="border border-primary rounded p-1 m-1">
                 <div class="dropdown btn-group" style="display: inline-flex;">
                     <i class="icon icon-common icon-search mr-1 " style="font-size: 1.75rem; vertical-align: middle" title="Text search"></i>                                
-                    <input id="q" value="{{ q }}" class="form-control" type="text" placeholder="Type search text here" onchange="modifyUrl('q',$('#q').val());">
+                    <input id="q" value="{{ q | default_if_none:"" }}" class="form-control" type="text" placeholder="Type search text here" onchange="modifyUrl('q',$('#q').val());">
                 </div>            
             </span>
             <span class="border border-primary rounded p-1 m-1">