changes testcase

freqtrade/strategy/interface.py

@@ -422,7 +422,7 @@ class IStrategy(ABC, HyperStrategyMixin):
             logger.debug("Skipping TA Analysis for already analyzed candle")
             dataframe['buy'] = 0
             dataframe['sell'] = 0
-            dataframe['buy_tag'] = ''
+            dataframe['buy_tag'] = None
 
         # Other Defs in strategy that want to be called every loop here
         # twitter_sell = self.watch_twitter_feed(dataframe, metadata)