fix: avoids call to DB when checking for queryset (#4552)

course_discovery/apps/course_metadata/models.py

@@ -1183,7 +1183,8 @@ def search(
             QuerySet
         """
         query = clean_query(query)
-        queryset = queryset or cls.objects.all()
+        if queryset is None:
+            queryset = cls.objects.all()
 
         if query == '(*)':
             # Early-exit optimization. Wildcard searching is very expensive in elasticsearch. And since we just