I’m not positive of the filename used, but you should be able to find it in the ApplyTransform transforms list, so if that’s what you’re seeing, then that’s probably right.
Are you sure that it’s all zeros? e.g.,
import nibabel as nb
img = nb.load(fname)
print("Zeros: ", np.allclose(img.get_fdata(), 0))
If so, I would inspect your input fieldmaps to make sure that they look sensible. All zeros suggests to me that you have the exact same data in your reverse-PE fieldmap, but this is just an intuition. I’m not as versed in the fieldmap components of the workflow as most of the rest.