fix warning log for Transformer SD3 (#8496)

fix warning log

src/diffusers/models/transformers/transformer_sd3.py

@@ -282,6 +282,7 @@ class SD3Transformer2DModel(ModelMixin, ConfigMixin, PeftAdapterMixin, FromOrigi
             # weight the lora layers by setting `lora_scale` for each PEFT layer
             scale_lora_layers(self, lora_scale)
         else:
+            if joint_attention_kwargs is not None and joint_attention_kwargs.get("scale", None) is not None:
                 logger.warning(
                     "Passing `scale` via `joint_attention_kwargs` when not using the PEFT backend is ineffective."
                 )