FIX: Incomplete transition to an HMC-free path

fmriprep/interfaces/resampling.py

@@ -539,7 +539,7 @@ def resample_image(
             classes = [xfm.__class__.__name__ for xfm in transforms]
             raise ValueError(f"HMC transforms must come last. Found sequence: {classes}")
         transform_list: list = transforms.transforms
-        hmc = None
+        hmc = []
 
     # Retrieve the RAS coordinates of the target space
     coordinates = nt.base.SpatialReference.factory(target).ndcoords.astype('f4').T
@@ -548,15 +548,12 @@ def resample_image(
     vox2ras = source.affine
     ras2vox = np.linalg.inv(vox2ras)
     # Transform RAS2RAS head motion transforms to VOX2VOX
-    if hmc is not None:
-        hmc_xfms = [ras2vox @ xfm.matrix @ vox2ras for xfm in transforms[-1]]
-    else:
-        hmc_xfms = None
+    hmc_xfms = [ras2vox @ xfm.matrix @ vox2ras for xfm in hmc]
 
-    # Remove the head-motion transforms and add a mapping from boldref
+    # After removing the head-motion transforms, add a mapping from boldref
     # world space to voxels. This new transform maps from world coordinates
     # in the target space to voxel coordinates in the source space.
-    ref2vox = nt.TransformChain(transforms[:-1] + [nt.Affine(ras2vox)])
+    ref2vox = nt.TransformChain(transform_list + [nt.Affine(ras2vox)])
     mapped_coordinates = ref2vox.map(coordinates)
 
     # Some identities to reduce special casing downstream