[Bug] Unable to generate chat title when `CUSTOM_MODELS` removed `gpt-3.5-turbo`

# Describe the bug
When `CUSTOM_MODELS` removed `gpt-3.5-turbo`,
Chat title will never be generated.

# To Reproduce
Steps to reproduce the behavior:
1. Set env `CUSTOM_MODELS=-gpt-3.5-turbo`
2. Deploy
3. Create new chat
4. Ask something
5. GPT will response successfully
6. but chat title will never be generated

# Expected behavior
Even if `gpt-3.5-turbo` disabled, the chat title should always be generated.

# Screenshots
POST - /api/openai/v1/chat/completions
HTTP Status: 403
```json
{
"error": true,
"message": "you are not allowed to use gpt-3.5-turbo model"
}
```

# Deployment
- [ ] Docker
- [x] Vercel
- [ ] Server

**Desktop (please complete the following information):**
- OS: Any
- Browser: Any
- Version: Any

**Smartphone (please complete the following information):**
- Device: Any
- OS: Any
- Browser: Any
- Version: Any

**Additional Logs**
Add any logs about the problem here.