Chatbot preview conversation does not work with guided conversation

Guided conversations do not appear to be working. When a user attempts to chat with the guided conversation, the conversation produces an empty response.

This empty response happens whether the graph does or does not have nodes. If the user should receive an empty response when the graph has not nodes (the default state), Delvin should make users aware that they must add nodes before communicating.

See Sentry Issue for additional information.

Steps to Reproduce

1. Log in
2. Create a new workspace.
3. Create a new project.
4. Enter project.
5. Click Add Assistant
6. Fill out the bot with default values. For model select on OpenAI key and "GPT-40 mini".
7. Click Next (to create the bot)
8. Click on the newly created bot.
9. Click Preview bot
10. Type hi

Conversations with the bot will fail.