Empty chats will also be saved to history.

**Describe the bug**
When creating a new chat and taking no further action, an empty chat entry is still recorded in the chat history.

**Versions**
- Copilot for Xcode 0.36.0
- Xcode 16.4 (16F6)
- macOS Sequoia 15.5 (24F74)

**Steps to reproduce**
1. Click the “New Chat” icon in the upper right corner.
2. Click the “Show Chats...” icon in the upper right corner.
3. You will see the newly created empty chat in the history list.