fix cross encoder conditinoal

app.py

@@ -47,7 +47,7 @@ def bot(history, api_kind, cross_enc):
     logger.info('Retrieving documents...')
     # Retrieve documents relevant to query
     documents = []
-    if not cross_enc:
+    if cross_enc is None:
         document_start = perf_counter()
 
         documents = retrieve(query, TOP_K)