Update simple_sentiment.py

simple_sentiment.py

@@ -14,7 +14,8 @@ class SimpleSentimentTool(Tool):
         "model_key": {
             "type": "string",
             "description": "The model to use for sentiment analysis",
-            "default": "oliverguhr/german-sentiment-bert"
+            "default": "oliverguhr/german-sentiment-bert",
+            "nullable": True
         }
     }
     # Use a standard authorized type