[fix] normalize time range parameter

searx/query.py

@@ -184,7 +184,7 @@ class SearchQuery(object):
         self.lang = lang
         self.safesearch = safesearch
         self.pageno = pageno
-        self.time_range = time_range
+        self.time_range = None if time_range in ('', 'None', None) else time_range
         self.timeout_limit = timeout_limit
 
     def __str__(self):