Prevent unwanted conversion to advanced search.

module/VuFind/src/VuFind/Search/Base/Params.php

@@ -449,13 +449,18 @@ class Params implements ServiceLocatorAwareInterface
 
         $this->searchType = $this->query instanceof Query ? 'basic' : 'advanced';
 
-        // If we ended up with a basic search, set the default handler if necessary
-        // and convert it to an advanced search:
+        // If we ended up with a basic search, it's probably the result of
+        // submitting an empty form, and more processing may be needed:
         if ($this->searchType == 'basic') {
+            // Set a default handler if necessary:
             if ($this->query->getHandler() === null) {
                 $this->query->setHandler($this->getOptions()->getDefaultHandler());
             }
-            $this->convertToAdvancedSearch();
+            // If the user submitted the advanced search form, we want to treat
+            // the search as advanced even if it evaluated to a basic search.
+            if ($request->offsetExists('lookfor0')) {
+                $this->convertToAdvancedSearch();
+            }
         }
     }