fix: fallback model_checkpoint if it's empty

[deanpress]

This fixes the following error when SD attempts to start with a non-existence checkpoint (e.g. after file removal):

Traceback (most recent call last):
  File "D:\Web\stable-diffusion-webui\launch.py", line 295, in <module>
    start()
  File "D:\Web\stable-diffusion-webui\launch.py", line 290, in start
    webui.webui()
  File "D:\Web\stable-diffusion-webui\webui.py", line 132, in webui
    initialize()
  File "D:\Web\stable-diffusion-webui\webui.py", line 62, in initialize
    modules.sd_models.load_model()
  File "D:\Web\stable-diffusion-webui\modules\sd_models.py", line 283, in load_model
    checkpoint_info = checkpoint_info or select_checkpoint()
  File "D:\Web\stable-diffusion-webui\modules\sd_models.py", line 117, in select_checkpoint
    checkpoint_info = checkpoints_list.get(model_checkpoint, None)
TypeError: unhashable type: 'list'