Fixup abstracts edge case detection

app.py

@@ -75,7 +75,7 @@ def format_response(neighbors, response):
         if title is None: # edge case: no title
             title = 'No title'
         
-        if abstract is None: # edge case: no abstract
+        if abstract_inverted_index is None: # edge case: no abstract
             abstract = 'No abstract'
         else:
             abstract = _recover_abstract(abstract_inverted_index)