this fixes a crash in the TI frontend (#2527)

- This fixes an edge case crash when the textual inversion frontend
  tried to display the list of models and no default model defined
  in models.yaml

Co-authored-by: Jonathan <34005131+JPPhoto@users.noreply.github.com>

ldm/invoke/training/textual_inversion.py

@@ -295,7 +295,8 @@ def get_model_names(self) -> Tuple[List[str], int]:
             for idx in range(len(model_names))
             if "default" in conf[model_names[idx]]
         ]
-        return (model_names, defaults[0])
+        default = defaults[0] if len(defaults)>0 else 0
+        return (model_names, default)
 
     def marshall_arguments(self) -> dict:
         args = dict()