Fix NameError in ZImageOmniPipeline when guidance_scale=0

`ZImageOmniPipeline.__call__` only defines `negative_condition_siglip_embeds`
inside a `if self.do_classifier_free_guidance:` block:

    if self.do_classifier_free_guidance:
        negative_condition_siglip_embeds = [
            [se.clone() for se in batch] for batch in condition_siglip_embeds
        ]

but later reads the name unconditionally when reshaping:

    condition_siglip_embeds = [None if sels == [] else sels + [None] for sels in condition_siglip_embeds]
    negative_condition_siglip_embeds = [
        None if sels == [] else sels + [None] for sels in negative_condition_siglip_embeds
    ]

`do_classifier_free_guidance` is defined as `self._guidance_scale > 0`,
so any call with `guidance_scale=0.0` raises `NameError`. This is the
exact configuration the pipeline's own `EXAMPLE_DOC_STRING` uses
(`guidance_scale=0.0` for the Z-Image-Turbo distilled checkpoint),
so running the documented snippet crashes.

The downstream consumption at

    condition_siglip_embeds_model_input = condition_siglip_embeds + negative_condition_siglip_embeds

is already guarded by `if apply_cfg:`, so we only need to guard the
reshape step to match. Wrap the negative-branch list comprehension in
the same CFG check, matching the symmetric treatment of
`negative_condition_latents` (which is already only defined when
`do_classifier_free_guidance` is true and used only in the CFG branch
of the denoise loop).

Author: Ricardo-M-L

src/diffusers/pipelines/z_image/pipeline_z_image_omni.py

@@ -588,9 +588,10 @@ def __call__(
                 negative_prompt_embeds = [npe for npe in negative_prompt_embeds for _ in range(num_images_per_prompt)]
 
         condition_siglip_embeds = [None if sels == [] else sels + [None] for sels in condition_siglip_embeds]
-        negative_condition_siglip_embeds = [
-            None if sels == [] else sels + [None] for sels in negative_condition_siglip_embeds
-        ]
+        if self.do_classifier_free_guidance:
+            negative_condition_siglip_embeds = [
+                None if sels == [] else sels + [None] for sels in negative_condition_siglip_embeds
+            ]
 
         actual_batch_size = batch_size * num_images_per_prompt
         image_seq_len = (latents.shape[2] // 2) * (latents.shape[3] // 2)