allow conversion from no state dict checkpoints

scripts/convert_original_stable_diffusion_to_diffusers.py

@@ -866,6 +866,8 @@ if __name__ == "__main__":
     else:
         print("global_step key not found in model")
         global_step = None
+
+    if "state_dict" in checkpoint:
         checkpoint = checkpoint["state_dict"]
 
     upcast_attention = False