Fix IndexError in CodeAgent.run method when message.content is empty

[codegen-sh]

Description

This PR fixes an IndexError that occurs in the CodeAgent.run() method when handling AI messages with empty content lists.

Problem

The error occurs when trying to access message.content[0] but message.content is an empty list:

IndexError: list index out of range

Stack Trace

File "/root/app/modal_app/gen/slack/handlers.py", line 46, in handle_app_mention
  agent.run(prompt)
File "/usr/local/lib/python3.12/site-packages/codegen/agents/code_agent.py", line 123, in run
  if isinstance(message, AIMessage) and isinstance(message.content, list) and "text" in message.content[0]:
                                                                                      ~~~~~~~~~~~~~~~^^^

Solution

Added a check to verify that message.content is not empty before trying to access its first element:

if isinstance(message, AIMessage) and isinstance(message.content, list) and len(message.content) > 0 and "text" in message.content[0]:

This prevents the IndexError when message.content is an empty list.

Testing

This fix should prevent the IndexError when the CodeAgent receives an AIMessage with an empty content list.

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