Fixed an unhandled error that was raised when DynamoDB did not have any chat history. (#6141)

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The current version of chat history with DynamoDB doesn't handle the
case correctly when a table has no chat history. This change solves this
error handling.

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Fixes https://github.com/hwchase17/langchain/issues/6088

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langchain/memory/chat_message_histories/dynamodb.py

@@ -44,6 +44,7 @@ class DynamoDBChatMessageHistory(BaseChatMessageHistory):
         """Retrieve the messages from DynamoDB"""
         from botocore.exceptions import ClientError
 
+        response = None
         try:
             response = self.table.get_item(Key={"SessionId": self.session_id})
         except ClientError as error: