Model: Add fallback if negative prompt is empty

Fallback to the BOS token since an empty string won't do anything.
Ideally, an empty negative prompt should not be used, but it's not
the end of the world.

Signed-off-by: kingbri <bdashore3@proton.me>

model.py

@@ -585,7 +585,11 @@ class ModelContainer:
         if cfg_scale not in [None, 1.0]:
             if self.use_cfg:
                 gen_settings.cfg_scale = cfg_scale
-                negative_prompt = kwargs.get("negative_prompt")
+
+                # If the negative prompt is empty, use the BOS token
+                negative_prompt = unwrap(
+                    kwargs.get("negative_prompt"), self.tokenizer.bos_token
+                )
             else:
                 logger.warn(
                     "CFG is currently disabled. "