fix: IndexError in CodeAgent.run method when message.content is empty (#821)

## 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:

```python
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.

Co-authored-by: codegen-bot <team+codegenbot@codegen.sh>

Author: codegen-sh[bot]

src/codegen/agents/code_agent.py

@@ -120,7 +120,7 @@ def run(self, prompt: str, thread_id: Optional[str] = None) -> str:
             if isinstance(message, tuple):
                 print(message)
             else:
-                if isinstance(message, AIMessage) and isinstance(message.content, list) and "text" in message.content[0]:
+                if isinstance(message, AIMessage) and isinstance(message.content, list) and len(message.content) > 0 and "text" in message.content[0]:
                     AIMessage(message.content[0]["text"]).pretty_print()
                 else:
                     message.pretty_print()