Fix ONNX img2img preprocessing (#1736)

Co-authored-by: Peter <peterto@users.noreply.github.com>

src/diffusers/pipelines/stable_diffusion/pipeline_onnx_stable_diffusion_img2img.py

@@ -332,6 +332,9 @@ class OnnxStableDiffusionImg2ImgPipeline(DiffusionPipeline):
                 f" {type(callback_steps)}."
             )
 
+        if generator is None:
+            generator = np.random
+
         # set timesteps
         self.scheduler.set_timesteps(num_inference_steps)