Fix for possible None in context.

src/evidently/features/openai_feature.py

@@ -78,6 +78,7 @@ def generate_feature(self, data: pd.DataFrame, data_definition: DataDefinition)
             context_column = repeat(self.context)
 
         for message, context in zip(column_data, context_column):
+            context = "" if context is None else context
             prompt_answer = func(
                 client,
                 model=self.model,