Fix prompt incorrectly set to empty when suffix is empty string
diff --git a/modules/chat.py b/modules/chat.py
index c8516c5940..5fb5523ca0 100644
--- a/modules/chat.py
+++ b/modules/chat.py
@@ -161,12 +161,14 @@ def make_prompt(messages):
 
             prompt = instruction_template.render(messages=outer_messages)
             suffix = get_generation_prompt(instruct_renderer, impersonate=False)[1]
-            prompt = prompt[:-len(suffix)]
+            if len(suffix) > 0:
+                prompt = prompt[:-len(suffix)]
 
         else:
             if _continue:
                 suffix = get_generation_prompt(renderer, impersonate=impersonate)[1]
-                prompt = prompt[:-len(suffix)]
+                if len(suffix) > 0:
+                    prompt = prompt[:-len(suffix)]
             else:
                 prefix = get_generation_prompt(renderer, impersonate=impersonate)[0]
                 if state['mode'] == 'chat' and not impersonate:

While using the qwen model, I use the template from its `tokenizer_config.json`.

```
"{% for message in messages %}{% if loop.first and messages[0]['role'] != 'system' %}{{ '<|im_start|>system\nYou are a helpful assistant<|im_end|>\n' }}{% endif %}{{'<|im_start|>' + message['role'] + '\n' + message['content']}}{% if (loop.last and add_generation_prompt) or not loop.last %}{{ '<|im_end|>' + '\n'}}{% endif %}{% endfor %}{% if add_generation_prompt and messages[-1]['role'] != 'assistant' %}{{ '<|im_start|>assistant\n' }}{% endif %}"
```

In this case, the `suffix` is an empty string, which len is 0.

The execution of `prompt[:-0]`, will incorrectly set the prompt to an empty string.

The conversation works fine, but use "continue" will get an error.

## Checklist:

- [x] I have read the [Contributing guidelines](https://github.com/oobabooga/text-generation-webui/wiki/Contributing-guidelines).
