bug fix for book search tool

tools/book_search.py

@@ -33,7 +33,7 @@ class NYTBestSellerTool(Tool):
         'genre': 
             {'type': 'string', 'description': 'The genre to search'},
         'limit': 
-            {'type': 'integer', 'description': 'The number of results to include'}
+            {'type': 'integer', 'description': 'The number of results to include', 'nullable': True}
         }
     output_type = "string"