Fix wrong do_classifier_free_guidance threshold in ZImagePipeline (#13183)

Z-Image uses CFG formula `pred = pos + scale * (pos - neg)` where
`guidance_scale = 0` means no guidance. The threshold should be `> 0`
instead of `> 1` to match this formula.

Co-authored-by: Hezlich2 <typretypre@gmail.com>

Author: kirillsst

src/diffusers/pipelines/z_image/pipeline_z_image.py

@@ -276,7 +276,7 @@ def guidance_scale(self):
 
     @property
     def do_classifier_free_guidance(self):
-        return self._guidance_scale > 1
+        return self._guidance_scale > 0
 
     @property
     def joint_attention_kwargs(self):